How can a broker invest our money if the money is kept in a bank’s custodian account to protect our funds in the event it goes bankrupt?

I don’t understand how that is possible, this is especially true for StashAway’s policy in the event they go bust.

Is it right to say that the broker has to match the amount of money we invest to keep our invested money in a Bank’s custodian account to protect us? and that technically they could be investing with their own money and vice versa?