GoodNotes (iOS)
Used by 6 peoplegoodnotes.com ↗

“I use an app called GoodNotes on the iPad to draw and design pieces.”
Moegi Yamanashi Interview, Jeweller · via Uses This 
“I've also found myself enjoying taking notes with the Apple Pencil in GoodNotes (the best fit for me of all the iPad apps I've tried) but I'm constantly on the lookout for a better note-taking solution.”
Leonard Lin Interview, Independent technologist · via Uses This 

“GoodNotes on iPad incomprehensibly does not have a tool that lets you make squares (you need to draw the square by hand and hope that the software magically recognizes that it was a square and straightens it up for you).”
Julia Evans Interview, Developer (Stripe), cartoonist, zine maker · via Uses This