Correct me if i;m wrong, but can some onhit effects crit?

I don't believe so, although built-in crits can crit so shrugs

Crit only applies to the listed AD of a champion when they auto attack unless something else is specifically stated like Garen's spin or the magic damage from Shiv.

If you have 150 AD with ferver, bloodrazor, bork, and wits end, a crit will do 300 damage and then the other effects will be added after. The same can be said of sheen procs.

In the same case as above with 100 base AD, the crit will multiply damage to 300, the sheen will multiply ONLY THE BASE DAMAGE by 2 to add 100 damage making the total damage of the attack 400. Then all other on hit affects will be added afterwords.