Question about sensitivity + Targeting mode

So I have been playing for a couple of weeks now on PS4 and wanna try to improve my aim. So far it is very sluggish. Imma tweaking some settings currently and have 2 questions:

1. I see "targeting mode" with 'Assisted aim - partial' being default. Can someone explain me the difference between this, Assisted aim - full, free aim - assisted and free aim?

2. On what sensitivity do you guys play (how for did you set the bar?). Is it smart to keep the aim-sensitivity very low or should i put it very high?