Question concerning buffs/debuffs

Hello, I just read the buffs/debuffs guide on MeowDB but I still have a question : How does buffs/debuffs effect duration work exactly ?

It's said that Ultimates buffs/debuffs stack in turn and duration (so, if I understand correctly, if you already have 20%+15% PATK buff fur ultimates (capped at 30%) and add 2 turns with a 15% ultimate, it adds 30% for 2 turns), but how does it work for regular skills ?

If you get 20% PATK buff for 3 turns and 15% for 5 turns from 2 skills in the same turn, do you get 30% for 5 turns or 30% for 3 turns and 15% for 2 turns ?

Also, is there a way to see the exact values of buffs/debuffs during battle or do you have to remember it yourself ?