Does a cantrip only count as “cast a spell” then?

Does a cantrip only count as “cast a spell” then? A caster can’t use a cantrip on an opportunity attack?

If my temporary HP are 10 and I take 30 damage, what is the DC of my Concentration Check?

Is there a list of animals Druids can shift into? Or just what’s in the PHB?

Can Dispel Magic dispel potions’ effects that are named like spells? And other potions?