Okay, there are two specific ways to accomplish this.
The first way assumes that the person starting the meeting does not need access to any Host functions,. This is called "Join Before Host". This requires the use of a Passcode and disabling the Waiting Room. Simply set it up to be joinable 15 minutes in advance of the scheduled start time, and anyone that has the Join link can join the meeting. See yellow highlights here:
The second way is if your substitute host does need access to Host functions, like starting the Cloud Recording, enabling the Waiting Room, etc. First you have to enable Join Before Host as above. Then there is one more set of steps. You give your substitute host your "Host Key", and that person uses the Host Key to "claim the host role" once they're in the meeting.
Complete instructions on locating the host key, editing it if appropriate, and how to claim host are available in this Zoom Support article:
https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0067063 which essentially says:
Your Host Key is locate on your Personal -> Profile page:
Click the "eyelash" (yellow highlight) and the asterisks will reveal your Host Key. There is an Edit button to the far right (not visible in this screen shot) allowing you to change your Host Key.
The substitute host enters the room, clicks the Claim Host button on the Panelist window, and enters the Host Key in the space provided:
