Xournal++ (a quick look at my setup)
Xournal++ is a free, open-source handwriting and annotation app that's been a huge part of my workflow for a while.
In this video I give a quick intro to what it is and walk through some of the customizations I've made to fit how I actually use it.
btw it works on Linux, Windows, macOS
Xournal++: https://xournalpp.github.io