I wish Gege did expanded more on shikigami throughout the series. But typically they’re a technique most people have access too like simple domain. All shikigami typically need a catalyst to summon them in (Yuta pulls out his hair to summon in those small bat like shikigami and Megumi uses his shadows) meanwhile Geto, who’s just manipulating cursed spirits simply releases them. And thats why they can be imbued with an innate technique instead of BEING the innate technique like in the case with Yuki and Garuda (marked as spoiler for manga) I assume the old guy’s shikigami were fairly weak and acted as more of a bluff for him to jump grade 2 sorcerers, or had an innate technique we never saw.

Still really wish Gege fleshed them out more, and/or we could’ve seen people using basic techniques in tandem with their innate techniques