You could do this with the Glide Send Email action, or you could use Make. I would probably use Make, but that’s just my personal preference. It’s up to you.
Assuming that you’ve implemented something similar to the solution that I gave to your earlier question, then all you really need to do is extend the action sequence that triggers when a seat is deleted. All you’ll need a is a joined list of email addresses of those in the waiting list, and then either pass that to the Send Email action, or trigger a webhook and send it to Make. Actually, I’m not sure if the Send Email action will accept a list of emails. I almost never use it, so that’s something you’d need to test.
You may need some conditional logic so that the email is only triggered if the number of available seats has increased from zero to one. Otherwise you’ll be triggering an email every time a seat booking is deleted, even if there are plenty of available seats. Although logically, you probably wouldn’t have any one on the waiting list if that was the case. So maybe only send the email if the joined list of waiting emails is not empty - or something like that.
To get the direct link to the flight details page, you’ll need to use Deep Links. You should note that deep links can only be set from the published app - if you try to set them from the builder you’ll just get a link to the builder.