user1 created draft Node revision, user2 can see the revision when editing the published node

That sounds like expected behaviour to me

the revision created by User2 which has not been approved/published by admin, how come the User3/User4/User5.. can see it and edit it ? the User3/4/5/. should edit the published version and create their own revision.

There is only ever one draft - regardless of which user created it

Think of a scenario where an organisation has 2 editors.

Editor one is working on a draft and saves some (unpublished) changes.

Editor two then needs to edit, logically they build on top of Editor one's changes - not starting from the published state - otherwise you'd have endless conflicts at publish time - what is being published - editor one's changes or editor two's.

what you saying is just one type of scenario, think about the wiki type, every user should only able to edit the published version but not an unpublished draft version from someone they don't know.

I'm telling you how Drupal works. Its not a scenario, its how default and forward revisions work by default. Workspaces module in core (experimental) does make changes to how that works to have a 'multiple forward revisions' landscape, but those are still shared across multiple users - but unique to a workspace. What you're describing is a whole new approach that is basically 'per-user' workspaces. I don't know if anything like that exists in contrib. I wouldn't recommend going down the path of trying to create something like that from scratch either - given how long Workspaces module has been experimental and how difficult it has been to get it this far.