[oluwie]

Installing it means installing a bundle or a package, which would come with all of the ads and intricacies you want to get away from.

There are almost no applications (at least that I've found) where you can download the source code of the app, build it and then install it locally to your device (except for testing of course).

[pbmonster]

I've been using FDRoid for many years now, so maybe it changed, but the two important examples always where:

* OSMand, the main openstreetmaps client on android: the version on Google Play comes just with the basics, if you want extras, like the topographic height contour lines overlaid on your maps, which is important for mountain sports, you had to pay, even that data is free and the feature is GPL-3.0.

* Simple Mobile Tools, a huge software suite of little tools (address book, voice recorder, calendar, ect.): if you get it from Google Play, it nags you to buy the "thank you" app, even if you already bought it. They has also started releasing "Pro" versions of the tools, which have more functionality and for which you have to pay - again. Everything is GPL-3.0., and therefor free on FDRoid.