https://en.wikipedia.org/wiki/Reserve_requirement#Required_r...
Tier 1 capital is not cash reserves, it's risk-adjusted assets, which may or may not be made up of assets that are as liquid as cash.
What happens when someone wants to make a withdrawal?
Fractional reserve in general isn't really a thing in modern banking. The US still does have a reserve requirement but pretty much the rest of the world relies on capital adequacy ratios (some capital is usually stored as reserves in exchange settlement accounts but it doesn't have to be).
Basically, for assets less than $124 million, the reserve requirement is 3%.
For more than $124 million, the requirement is 10%.
In other words, for every $100 on deposit at a bank, it can create "out of thin air" $90 and loan it out.
If someone then gets that $90 and puts it in a checking account at the bank, the bank can create and loan out $81, then $72, than $64, $57.6, $51 etc.
[0] https://www.federalreserve.gov/monetarypolicy/reservereq.htm