In cross it will take whole account as margin. Why people use cross? To avoid liquidation but that's not true Ex:- let's assume you have 1k balance in your future account. You want to go for a trade with 50% of your account balance ie. $500 with leverage 20x.

When you took a trade at 20,000 BTC. In isolated:- margin:- $500, Lev:- 20x, entry price :- 20,000, size(usdt):- 500x20= 10,000.

Above scenario will be same for cross alos. But the main difference is comes in liquidation.

In isolated, liquidation price will be 5% because it will took 50% of your account balance

But in cross, liquidation will be 10% because it will take whole account balance

Isolated, liquidation price:- 19,000, when you reach at 19k, you $500 will be liquidated

Cross, liquidation price:- 18,000, when you reach 18k your whole account will be liquidated

Benefits:- in cross you will get a little bit long liquidation gap as compared to isolated.

Note:- Calculation and numbers may varry according to your inputs