Questions regarding user accounts / installing programs.

My last computer (xp) I used what I assume was the administrator account for everything (My everyday account). I’m told that this was wrong(bad) and I shouldn’t do that.

So now new computer, and I’m going from a one user, to setting up two users on the computer.

They way it was explained to me was to have 3 accounts.
1 for Administrator, this account with a password, would basically be left alone and nobody would use it. Except I guess I’d be the administrator. It would be a “clean” account in case something bad happens I’d have something to fall back on if needed.
Then create
1 user account for ‘user1’ or we’ll call it ‘my’ everyday account.
1 user account for ‘user2’ for another person.

Is this correct?

Which then brings me to installing programs.
On the old computer I’d just install all programs in my everyday account (admin).

Now I’ll have 3 accounts.
Some programs I want for one user, but not for the other, or some programs I want available for both users.
I understand not all software allows you to pick where you would like the software to be installed.
Which account(s) do I use to install programs.?

For instance I installed firefox using ‘my’ account but yet it shows up in the ‘user2’ account and the administrator account. While I needed firefox for both ‘user’ accounts I don’t necessarily need it in the administrator account.
I don’t really want to junk up the computer with unnecessary files and what not if I don’t need to.

Should software be installed using the ‘user’ account the software is intended to be used in.?
Or should I be installing all software in the admin account, which I would think defeats the purpose of leaving the admin account alone so that it doesn’t get contaminated. ?

And how do I choose which user(s) accounts the software will or wont be available for use. ?

Thanks