Who holds the shares when we buy through interactive brokers?

Your shares will be hold by IB. They are custodian for your shares.

Hey Anon! For IB, it is held with the brokerage account. This is why they can afford to give you such low commission rate. I would say they are probably the most worthwhile brokerage account for custodian. This is especially true when you want to trade stocks.

By custodian, you can imagine it as depositing your money into your friends account for safekeeping.

Hope this helps!