When the hotel API shows me a price, does it include all the taxes, or will my customer be asked for more money at the hotel?

Usually yes, the price your staff see in the hotel search already has the taxes the supplier collects built into it, so the amount your customer pays you covers the room and those taxes together. That is the figure your portal quotes and the figure your voucher is based on.

What sits outside that amount is the local charges some towns and some hotels collect at the front desk themselves, things like a city or tourist tax, or a resort or service fee charged by the property. The supplier cannot take that money on your behalf, so it is not part of the price you are shown, and your customer settles it directly when they arrive or when they check out.

The hotel API does normally send a note about those charges, and the portal shows it alongside the room so your staff and your customer can read it before the booking is confirmed. We should be honest that how clear the note is depends entirely on the supplier. Some send it as a tidy separate item that can be displayed on its own line, while others send it only as a sentence buried in the room conditions, sometimes in the currency of the hotel rather than yours, so it cannot always be presented to the customer as one neat total in advance.

Before you start selling, run a few test searches on cities that are known for charging a tourist tax, open the room conditions on a handful of results, and see how that wording comes through so your staff know what to tell a customer who asks.