I'm new so this question might be a bit silly for most but I'm really confused about some of those keyboard shortcuts in mods. A lot of mods have shortcuts on for example CTRL + F8. Pressing CTRL apparently does nothing because if I have another shortcut on F8 it always brings up both menus, for CTRL+F8 and F8. What am I missing here? The CTLR thing does work with other keys but not for F's. Any ideas what I might be doing wrong or where's the catch?

You'll have to review the readme for each mod you've installed to figure out which keybind does what, and if it is possible to rebind them if there is a conflict. 

What ineseri said, check the readme's and config files to see if they can be rebound.

Keybinds in GTA are not exclusive unless the developers have put in counter-measures to make them exclusive. So pressing CTRL + F8 will always trigger both the modifier action (CTRL) and the default action (F8 on its own). This is because a plugin may be detecting if F8 has been pressed in it's thread, and not care if any other key is pressed or what other plugins are detecting.

I see, so it depends on each plugin. I came across quite a few plugins that use the modifier CTRL with F-something combination but so far all of them also triggered the plugin that's on F-something with no modifier so I didn't (and in a way still don't) see a point of using modifier with F-keys. I guess it's trial and error type of thing than.

I already burned quite a few hours rebinding in configs but regarding modifiers and F-keys the result is always the same.

Yes that's right. As I said a plugin likely only uses a simple key check so it won't care if a modifier key is held down or not and thus even if you hold a modifier down that single key check will trigger.

Some of the developers have spoken about using exclusive key checks, i.e. if a modifier is held their single key check wont trigger (because it isn't the only key held down). However whether or not they have implemented it remains to be seen.