Wow, Leon!
That's a lot of questions, there!
Guess I'll be the first to punt back for you... And, hey, I'm more a noob myself than not- having only recently returned into the hobby after about 35 yrs elsewheres!
That said: Your questions.... many will be hesitant to start in on them, as practices and approaches differ widely both among modelers and among genres of models... Me? I build almost only 1/35 armour. So I'll offer some anwers from that perspective and these will certes differ widely from what many other folks do. YOU get to pick and choose!
Here's my process:
I like to do the build first- assemble large units and sub-assemblies. I paint later.
I generally keep the tracks and road gear separate from the hull, but cleaned and assembled for later attachment after the hull gets painted.
I generally clean up and attach almost all small details to the turret and hull assemblies before painting either those details or the hull.
In some cases, details such as tools and attached crew kit are held aside and painted separately; and particularly so when I need to paint the hull behind those details (the same as for behind the track-set and wheels of a tank or armoured car).
I often clean up seams on parts while still on the sprue- specially tiny ones- saves lost bits for me.
I am an air-brush fan, mostly. It's about the finish I can get with the AB versus the brush- particularly for larger surfaces. Painting the whole tank at one go keeps things more united.
I paint and weather interiors generally before I do the outside stuff.
I decal or spray on or hand-brush on insignia and numbers on the completed base-coated kit before attaching those last tools and details (which may lay over parts of decals, etc.)
I hand-brush all the attached small stuff (tools, kit, etc.) after completing the major base-coats and decals / insignia on the entire kit.
I paint track sets and road-gear separate from the body, and then assemble them on to the painted hull before weathering.
I weather the complete vehicle, with all details and decals attached (other than those things simply placed loose on the tank- loose gas cans, etc.).
There are other approaches, but this is how I generally do things!
Hope this helps you a little

!
Cheers! and Good Luck!
Bob