Booking.com are indeed just agents.
Hotels, irrelevant of what the blurb says on booking.com's site will all work differently, until you book you'll really have now idea if a)they'll take nothing in advance b)take a minimum amount to make sure your card is live or c) take the full amount.
In my experience this just isn't true. They usually give you different prices for paying up front, paying later or paying at the hotel. If you book a hotel with free cancellation, they will tell you what the latest cancellation date is, and assuming you haven't cancelled, will take payment in full a day or so after this cancellation date, or sometimes at the end of the stay. If the hotel needs a deposit, this is usually made clear, and also whether this is refundable should you cancel.
If you pay with a credit card you are buying on booking.com's terms and conditions, not the hotel's, so have protection. Take screenshots of the terms and conditions and receipts detailing payment terms in case these aren't clear in the confirmation emails, is my advice.
OP, if they are asking to
hold an amount prior to your arrival, or cancellation date, then you
shouldn't pay any interest on that. It's just an amount blocked on your card but isn't a transaction UNLESS they call it a deposit/pre-payment. It's worth noting these blocks can sometimes take a month to clear when cancelling, which can affect available credit. Have a read through the Ts & Cs associated with the room details, it should explain the hotel policy. Pretty sure if you hover over the question mark in the 'Your choices' column it will explain the procedures and timescales for payments.
It's useful to have your banking app on your mobile when travelling for monitoring credit card spend. Payments show up immediately these days.