I started playing Wot after a long time and I wanna try some arties and I kindly ask how the stun mechanic actually works?

If I underestood correctly, do I give more stun the higher my shell penetration is and it also depends on the enemy tank armor? (Less armor -> more stun; more armor -> less stun?) Or neither of those and I’m imagining things?

Edit: And why is there a separate flair for only arty? xd