Hello @MelodyMoy
QR for mobile login should definitely work on the canvas app. If it isn't working, try clearing your cache or even deleting the app completely and redownloading it.
https://community.canvaslms.com/t5/Canvas-Student-iOS-Guide/What-Student-app-options-can-I-manage-fr...
Additionally, some things just aren't going to work on the app like Jigsaw or other external apps. Some apps are integrated and do work well within the canvas app but many will not work. It's like trying to open an app (Excel) within another app (facebook) it just doesn't work that way really and a lot of apps are designed for that purpose only. In which case, you'd need to use a browser for all of these external tools. Hopefully that helps.