At the point I make a loan, 2 things happen on my balance sheet: I have a new liability to you (the increased balance in your account), and I have a new asset (the loan that you’re expected to pay back). They cancel each other out and it therefore seems as if I’m creating money out of thin air.
However, the moment you actually use that money (eg to buy something), the money leaves the bank (unless the other account is also at this bank, but let’s keep it simple). Liabilities on the balance sheet shrink, so assets need to follow. That needs to come from reserves because the loan asset keeps its original value.
The reserve comes from the bank, not from you. Added layer here: Banks can borrow money from each other or central banks if their cash reserves runs low.
Finally: it tends to be the case that the limit on lending is not the reserves, but on the capital constraints. Banks need to retain capital for each loan they make. This is weighed against the risk of these loans. For example: you could lend a lot more in mortgages than in business loans without collateral. Ask your favorite LLM to explain RWAs and Basel III for more.