What is In-Game Shooting Mode Switch????...

I love how this game doesn't explain anything, ANYWHERE! What is the In-Game Shooting Mode, and what am I switching between?!

And also, is there auto-aim, because I don't believe I've EVER experienced it in this game. Call me a newb, but I think all 1st person shooters should have that, as we aren't able to control the characters with the same fervor that we can our own bodies (not to mention that a host of senses are obsolete, making shooting this way unreasonable).

Since you're on PC, no there isn't in-game shooting mode switch. It's only available on mobile where you can toggle between auto and manual fire.
As for the aim assist (not auto-aim, that's a hack), again only mobile has it. PC doesn't have it because you normally have more precision with your mouse than with your finger dragging on the screen.

But really, if you know you are a "newb", the first thing to do is play more and improve instead of asking for ways to make things easier. Tons of people out there don't need it nor do they want it, so why should aim assist be available in every FPS?