Exchange currency in app or during the transfer?

Hi, I can’t find my exact query so perhaps my query is oddly worded.

I am travelling from UK to US in April. I am planning to exchange GPB to Dollars on the app.

I can use any method to transfer but it doesn’t seem to like Apple Pay so I’ll use my debit card as it’s slightly easier than doing a bank transfer.

Does it make a difference if I transfer funds in as GBP first then convert to US in app after, or, if I select to bring it in as US right away? I.e. bring over 5/600 GBP then convert to US and see what I get, or, bring in 5/600 US and it will just take the require amount of GBP pre exchange from my bank.

Unsure if second method would use bank exchange rate instead of the revolut exchange rate.

I assume it’s better to make a large exchange in advance, rather than just keep all funds in my local currency and let the exchanges happen with each transaction?