There is a bunch of apps on F-droid (and also playstore) which start with "simple..." all opensource. My list: simple calendar pro, simple contacts pro, simple dailer, simple draw pro, simple filemanager pro, simple gallary pro, simple notes pro, simple SMS messenger. No ads on any of them, and free. I have a total of 39 f-droid (so opensource) apps on my phone... So start with F-droid
themadcodger