One of the problems with FCPX is how they decided to name the components in my opinion. The nomenclature isn't the best. Apple could have been much clearer in naming these things, it's something that comes up again and again when I teach FCPX. Also, yeah, it's not obvious that you need to Double-Click some things but not all INSIDE an application, like Projects require a double-click but Events and Libraries only need a Single-Click. It trips up people time and time again.
Library > Event > Projects is a bit misleading. I like to think of it and explain it as when I teach friends as:
Client > Product > Commercials (or Videos or Video Edits)
Library is the Client, Event is the Product, and Projects are the individual edits/videos
One of my friends thinks of it as Band > Album > Music Videos
I tend to keep one Library for each Client. For example:
Car Company > Car Model > Car Commercials
A "Project" is what was known as a Timeline or Sequence (it's the video you're editing) and Projects live inside Events. So while you can keep a Project for anything in any Event, it's good practice to keep the Project (the edit, timeline, sequence, whatever you prefer to think of it as) inside the Event for its subject.
(Library) Car Company > (Event) (2021 Model Sedan) > (Projects) (2021 Model Sedan 60 sec cut, 2021 Model Sedan 30 sec cut, 2021 Model Sedan 15 sec cut)
There is no "Master" timeline, rather each Project (created with the key combo Command N) is a different edit, a different video project. You can have as many Projects (video edits) as you need. It's not good practice to delete the contents of a Project (an edit) and re-use it. Rather it's better to create a new Project (edit, timeline, sequence) for each video you want to create. This way you retain access to them for future re-use, changes, etc...
The only "gotcha" is how FCPX handles Compound Clips (they're like Nested clips in older FCP 6 and 7). Each Compound Clip links to a "Parent" compound clip and so if you copy/paste a Compund Clip to another Project, then change it, ALL other instances of that Compound Clip will change. If you don't want it to you have to Clip > Reference New Parent Clip. It can be tricky to keep up with many compound clips but they can be VERY handy for making and reusing title cards and things.