About this Mac > Storage > Manage :

Refer to Optimize storage space on your Mac.

Excerpt (with emphasis): "When space is needed on your Mac, macOS also clears caches and logs that are safe to delete, including temporary database files, interrupted downloads, staged macOS and app updates, Safari website data, and more."

macOS does all of that automatically, even if you do not choose to "optimize storage".

For example, if on opening an item, a tab appears

that says that you need an app from the

Apple store to open the file (or one on your computer)

is that a sign that you can delete that item, if you don't

That requires more explanation. Can you provide an example? Describe the item you are opening. You generally do not need to obtain anything from the App Store.

Yes. For example you have to specifically select Optimize... if you want to allow automatic removals of iTunes Movies and TV shows you already watched:

Those movies and TV shows will remain yours to download again. They just won't be occupying space on your Mac. Movies and TV shows typically occupy a lot of space.

Those other temporary files—files Apple describes as "caches, logs, temporary database files, interrupted downloads, macOS and app updates, Safari website data, and more"—are deleted automatically, but only when your Mac needs the space they occupy. For them, there is nothing you need to do.

Where does one deselect "optimize storage" to allow the automatic removals and deletions?

Excerpted from that same Apple Support document,

• To view recommendations and decide how to optimize storage on your Mac, choose Apple menu  > About This Mac, click Storage, then click Manage.

None of that is required though. If a Mac is running low on available storage, it will let you know. If it should ever get to that point, don't ignore the warning.

I see this video as indicating that ...

No. Never do that. Pay no regard to YouTube and the dolts who inhabit it.

This is what I

would like an opinion on, before I do anything.

Never in the 30+ years of using Macs have I ever intentionally deleted a cache file. Not once, ever. That's one reason they all work as well as they work and last as long as they last. Should a Mac begin to exhibit strange behavior a "Safe Boot" deletes certain cache files maintained by the system, which often resolves that behavior upon a subsequent, normal restart.

The other reason is that I don't install any non-Apple "anti-virus", "cleaning", or "Internet security" junk. Those things account for the overwhelming majority of Mac miseries reported on this site. I can only speculate that Apple, having become quite understandably shocked at the prevalence of such garbage, responded by incorporating certain features those products claim to offer, except they're doing it with knowledge of Apple systems that is exclusively Apple's.

Cache files exist to maximize a Mac's performance as well as its economic life. Leave them alone. If you are having trouble with your Mac, please describe what it is in as much detail as you are able to provide.

If you are not having trouble with your Mac, then don't ask for trouble. Macs are designed to be treated with benign neglect. Use them when needed, ignore them when not.