Id suggest that you start by creating a trivial add on, and get that working first. Then gradually build up to what you eventually want to produce as it going to be pretty complicated - which means there are a lot of thing you will need to learn before you get it working.
Jumping right into a high-complexity project from a scratch start is like trying to learn to drive in a F1 race - and about as likely to succeed. All you will do is crash and burn, and that is not a good encouragement to continue a dev career. Start small, learn well, build your skill, and move up in terms of complexity.
And do yourself a big favour: don't bother with YT videos. Most of 'em are there for likes and subscribe, and the creatos have no idea how to teach, how to make a video, and in most case how to code either. Money rules in YT: quantity pays, quality takes time - so they don't bother with that bit ...
Start here:
Extending Gmail with Google Workspace Add-ons | Apps Script | Google for Developers[
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