I was not the host.

I booked a zoom video conference a day in advance, but by time the next day, I entered the conference, and I really found out that I was not the host.

Welcome to the Zoom Developer Forum, @ma.qianli! We are happy to have you in our community. First, can you confirm whether you are joining from the client or SDK? Based on the described behavior, it sounds like you may have joined the meeting as a participant. If you are looking to Join as a host, you will want to leverage the Start URL returned in the Get and Create a meeting API response. See an example Get Meeting Response here for reference:

Get a meeting

To learn more different options available to join a meeting, see our support Starting or joining a meeting as the host support documentation below :

Starting or joining a meeting as the host