So, a customer takes a loan, and with that money pays for a house. The house owner gets that money (usually from the bank, directly, although that money passes briefly through your pockets as well). That's actual money that is in circulation.

So far correct.

Now, you have to pay back to the bank what you took from it, over 30 years, with interest. The bank indeed keeps the interest, but the money you pay to it is going back into their reserve - from where they took what they gave to the house owner.

No. That money does not go into the reserve. It ceases to exist.

When a loan is created, the money does not come from any reserve. It is created out of thin air. When the principal, which was created when the loan was issued, is paid back, it is removed from circulation. It goes to the equivalent of dev/null.

The reserve exists for the purpose of covering bad loans that do not get paid back. I'm not exactly sure where the bank gets that money but the important part is that they're required to maintain a minimum ratio of reserves to loans issued.

What this means is that the part of the money supply that get created by commercial banks by issuing loans, can grow or shrink depending on the ratio of new loans issued to old loans paid back.