Where can I install Air Tags

Yes and yes.

But to get good performance, they should be on items you’re likely to leave behind in “regular” locations (e.g. home, office) or inadvertently leave in “irregular” locations w/ other iPhones.

An AirTag w/o a nearby iPhone to “find it” will remain un-locatable.

If I can summarize the use of AirTags into categories, I would offer the following, based on their usefulness:

• Great: misplaced or lost items, like keys
• Good: just about everything else
• Fair: travelling items not always with you, like luggage << AirTags rely on the Apple "Find My Network" when your iPhone is not within Bluetooth range. As such their location data can be sporadic or not accurate.
• Poor: Stolen items / Tracking pets or people << Apple did not design them for tracking. They can easily be disabled. They can be annoying to pets or people when they emit a sound.

Yes, absolutely agree!

I’d add that the “left behind” feature (for items normally carried with you) is also quite useful and reliable.