The style, and especially the splotchy background coloring, made Paper[1] (By WeTransfer it seems? When it was released they were called fiftythree.) my first guess.

[1]: https://apps.apple.com/us/app/paper-by-wetransfer/id50600381...

Looks like they used excalidraw.

It’s a OSS and a PWA. A lot of people seem to use it because it lets you draw without worrying about details.

It could also be an Onyx or ReMarkable tablet as well.