Wy wife's not an illustrator - she's afatchematician, so for her the essential app on iPad is Goodnotes 6, and it syncs across to MacOS automatically ( a single license allows you to use the app over all platforms). It's not a professional illustration app though, it has SOME drawing tools but it's more aimed for notes, presentations, brainstorming, online meetings etc, - a good balance of handwritten text, text converted into "text", doodles, quick graphs etc. It's also quite customisable.
For more accurate drawing / illustration work, ProCreate is THE app on iPad, but there's no MacOS version. :-(. You could draw in ProCreate and then export to a different product on the MacOS, but I guess you're already swopping between apps already, so this isn't the perfect solution for you.
Sketchbook is on both iPadOS and MacOS, it's not as fully featured as ProCreate, but it does work seamlessly across the platforms.
Away from specific use-cases, I'd argue that, if you use any iPad with any pencil, and also use a Mac, Goodnotes is something you should have on both devices, just because it's very handy to have.