Suggested Views and Ranking

As far as I understand You get suggestions in side bar (or in bottom - mobile) while watching video - Now to get in that spot, we should use the same tags as he does right?

But what if these are the tags / keywords that people does not search at all?

The best practice is to target keywords that has both search volume and suggestions from similar video?

Most of the time relevancy is the most important thing when we speak about suggested video.
There are cases when suggested videos shows videos that target user past history of watching.

Tags are not the only thing that gets you in suggested list, title play an important role.
You can see a lot of videos that don't use tags and still appear on suggested videos because of title and relevancy.

It's logic that you want to target keywords that has a decent search volume otherwise you will just waste your time.
Best tactic will be to use keywords that are 100% relevant for your video and good thumbnail to have high CTR.
If your video gets views and engagement it will get suggested more often.