HTB ISA - what can I actually use the bonus for?

So my boyfriend and I both opened help to buy ISA’s in November and have desposited £1200 each initial, and a further 200 each this month. We will continue to put in £200 each month for around the next year, possibly longer.

My question is this: I have been told that you do not get the bonus until completion of the house sale, meaning you cannot use it as a deposit to the seller, as the funds will not be available until completion of the sale. Therefore you can only use it as part of the deposit on your mortgage. Is this right?

Also can some please explain this to me - I thought you paid one lump sum deposit, I didn’t realise they were split. I’m really confused and am not getting the information I need online or podcasts etc.

Any info would be greatly appreciated. Thanks so much.