Hallo, the first quastion hasnt a real answer. Everything depends on your needs, so I dont believe you can write a lot (poor precision and speed, so you have to write quite big), but boox is excellent to scribble.
You can also be interested in converting your books in .djvu, which is much faster and has a better quality (
https://wiki.mobileread.com/wiki/DJVU) but has some limitations.
In any case a new hidden folder is created, so you simply have to save it. But this sketches are readeble only by boox.
Pdf can export your annotation in a new pdf readeble on any computer, djvu hasnt this feature.
bye!