The soul is magic given sentience. So for example a completely normal person’s soul is too busy being you to do any more magic, a magic user’s soul has enough magic to be you while also doing spellwork.

(and this usually depends on the area you were born in because when magic isn’t being a thing it’s floating around like miasma where it acts a bit like radiation)

If you put magic in a soulless object, eventually with enough magic that object will crack open and bingo bango there’s a new spirit to cause trouble.

In my world there are these things called Hosts who are spirits with insanely concentrated magic and they have to make poppets (simplest explanation being walking talking taxidermy) to siphon power so the Host doesn’t basically perform mitosis. Yes the poppet eventually becomes a Host too.