There is no skill point cap in ESO. You can, through much effort, learn all the skills you want.

How would passives work then? Does that mean that you could eventually get every passive from your class?

You can eventually get every passive from every tree. Once you have collected every passive that doesn't mean you will be OP. Passives are always tied to something else.
For example, there is a passive that enhances your abilities while wearing heavy armor. That passive won't work while you are wearing all light armor. If you mostly wear heavy but throw a few pieces of light armor in there, it won't be as potent as if you were wearing all heavy. Likewise there are passives that increase your mana regen while wearing light armor.
There is a passive that enhances your abilities with daggers. The passive won't benefit you if you don't have any daggers equipped. There is a passive that adds a snare to fire type abilities. Very helpful if 5 out of your 6 action bar slots are dragon knight abilities that happen to be fire orientated. Not as helpful if only 1 of your 6 action bar slots happens to deal fire damage.