it seems that Shadowheart who is a cleric and not a thief has a better chance of opening locks or disarming traps than even Astarion whose the rogue; and I always thought for things like that the rogue is the best way to go.
Not in 5e, not anymore. Classes are much more malleable, and not so strictly separated. In this case, DEX modifier + Sleight Of Hand proficiency is all that matters (oh, and the Guidance cantrip helps a lot). Since Shadowheart has Urchin background, it gives her the aforementioned proficiency; plus, her Dexterity is so abnormally high for a cleric that it's possible to use her instead of a Rogue (I do). Until / unless Larian will implement proficiency in tools and / or skill expertise, she is essentially on par with Astarion as far as locks and traps are concerned.
Also, the disarm tools which I thought were used to disarm traps what good are they? As shadowheart has been disarming traps and didn't have one in her inventory.
In BG3, inventory is... stretchy. She "uses" disarming tools if anyone in the party has them. Same with keys.
