how Bitcoin replaces Swift and becomes a Global reserve currency?
Internet's most comprehensive step-by-step explanation

Modern banking is complicated to say the least, there are a whole bunch of things that go behind the scenes that make up the modern digital payment infrastructure that the world is so reliant on.
When you send money to someone in say Argentina, it usually goes through what is called a SWIFT transfer
SWIFT - Society for Worldwide Interbank Financial Telecommunications is the world's largest financial network that moves the world's money.
Last year SWIFT moved an insane $5 Trillion per day or 1.25 quadrillion per year most of which is international payments.
let's start from the basics!
when you enter the details of your Argentinian friend in your bank account and authorize a wire transfer, A whole bunch of banks called correspondent banks get involved to make this go around ( https://www.thebalance.com/correspondent-bank-5198871 )
your bank sends these details to a larger 'correspondent' bank which then sends this to another larger bank which has a relationship with an Argentine bank.
Since there is a currency conversion involved as there might be one more bank involved

This in fact is a simpler process, a lot of times when business customers are involved things get far more complicated.
Trust Issues
Just the way you trust your bank to keep the funds safe, your bank trusts the larger correspondent bank to keep its funds safe during transit ( which means this larger bank needs to keep a huge deposit safe with the federal reserve to maintain its credibility)
So to simplify things, since everyone needs to trust one another every step of the way, only select entities which have huge capital pledged with authorities can be part of this trust chain. As you can expect these trustworthy entities don't want to do the job for free, so they charge for this privilege.
So the bills quickly pile up for every entity that is involved in the food chain. Even though transferring money in the modern world is simply moving numbers from one database to another, transfers between countries typically cost 5-10% ( for most corridors ) and take 1-5 days.
Part of the reason is also exacerbated by the fact that the dollar is the preferred transit for every currency transfer ( i.e a rand-to-yen transfer would involve a rand-to-dollar purchase first and then a dollar-to-yen sell after ) and usually the banks dealing with dollars have far higher regulatory requirements which delay the process significantly.
The Problem in conclusion
International payments cost too much ( 1-10% ) and take anywhere between 1-5 days to sometimes as high as 30 days.
The Blockchain Solution
As Blockchain decentralizes trust and makes payments instantaneous using a blockchain-based token like Bitcoin even stable coins like USDT or USDC bring down the costs drastically to a few cents and time taken to minutes.
So why haven't international payments switched to Bitcoin or USDT already?
Switching costs and trust
You see when businesses want to import /export goods worth millions there is a significant legacy supply chain that works seamlessly with existing payment infrastructure. The second reason is trust, trusting a decentralized blockchain system to move money around takes time for people to accept it won't happen overnight, will take years to decades.
The good news, many small ticket remittances across borders are already happening via stable coins and many times through bitcoin

But slowly and surely the global business community will adopt Bitcoin / stable coin-based settlement owing to incredible advantages.
Imagine a business that uses Bitcoin-based settlement and saves 3% in fees which then passes to its customers and makes more profits.
Again all this will take time when it comes to large businesses jumping on the bandwagon.
Bitcoin to the rescue?
Now imagine that your bank didn't have to send money to another bank which sends money to another bank which sends to another bank, you get the point, Instead, your bank could simply purchase bitcoin in its own country from an exchange and send this bitcoin through bitcoin lightning (at speed of light) which gets converted from bitcoin to that local currency to your friend ( all of this can happen in less than 5 secs )
The powerful thing about bitcoin is it's the world's first digital bearer asset which doesn't need a trusted counterparty to move funds from bank to bank. Even though bitcoin is volatile in a window of months over a 5-second window that volatility won't affect you much.
Using this route you can basically zap money around the world in seconds and cost l/1000 of typical SWIFT transfers.
Liquidity Begets Liquidity
When you use bitcoin lightning you get the liquidity of the bitcoin network which is the highest compared to any other crypto asset out there, which means money zapping around from currency to currency costs much less if you were using bitcoin instead of other cryptos. There is an interesting after-effect as more money gets zapped around through the bitcoin network liquidity improves further, along with an increase in price as there is only to a limited amount of bitcoin to around ( unlike dollars which are practically infinite).
Eventually, there will come a point where using any other network for transferring money across the world simply wouldn't make sense as Bitcoin's lightning network can scale to 100s of millions of transactions per se and offers the best liquidity compared to other assets.
So, why are stablecoins no replacement for bitcoin?
The issue with stablecoins is that they work fine for retail users and small ticket size transfers however the moment a sizeable amount needs to be transferred ( say $1M or more ) a lot of regulatory bells will ring which adds up the overhead of 'Dollar' oriented regulations.
The other major issue with stablecoins is counterparty risk, the entity that is issuing stable coins like tether etc can always print more tether than actual dollars it has in the bank. With Bitcoin, however, there is no such risk as bitcoin has no counterparty risks like stable coins and over time the liquidity offered by the Bitcoin lightning network will be so much higher than moving money via the bitcoin network would cost lesser than even using stablecoins.
The biggest issue is not even counterparty risk or regulatory compliance it's cost. You see the banks are for-profit entities, not a charity, so their investors need to buy a bunch of stablecoins to provide this service and they will be not earning any interest ( as stablecoins in transit don't earn interest ) which means the banks will have to charge substantial fees for this service.
On the other hand, Bitcoiners are happy to hold Bitcoin without any expectation of interest as Bitcoin supply is limited ( no one can print more Bitcoin unlike dollars which are getting printed to infinity ). A trillion-dollar payment transfer mechanism can be handled on Bitcoin simply by using the liquidity of Bitcoin holders.
Not a pipe dream
Companies like Strike ( which is in the business of lightning-based payments and just raised $80 Million to do so ) are already working their way up to move some of the payments through the Bitcoin lightning network.

The Reserve currency
As more and more businesses around the world start using Bitcoin for payment settlement initially, they will realise that it's actually a good idea to hold some as a store-of-value so that they can actually avoid conversion on one leg i.e conversion from local currency to Bitcoin. Once companies start making significant profits on their Bitcoin Positions, others will follow suit pushing the price up further which will slowly but surely happen as the dollar prints its way to infinity.
Conclusion
Even though comparing the dollar to bitcoin looks like David vs Goliath the basic realities of market forces favour Bitcoin.
No one likes to wait 5 days for payment clearance if they don't have to.
No one likes paying 1-10% of money transfers as transaction fees.
No one likes giving up their privacy to agencies in countries that they are mostly not even aware of.
All this means is simple, when compared to SWIFT, Bitcoin is the winning horse.
As Bitcoin replaces SWIFT, it will slowly but surely gain the reserve currency status
.