Our public library has the function to borrow e-books (called onleihe, but this counts only for some libraries in germany). I use this a lot.

I think it is, especially when the default doesn’t communicate well with a different ecosystem (Android vs iOS). So installing a app that gives you more features than plain text and works with all relevant mobile os worlds absolutely makes sense.

I am from Germany

I would presume so. Both contain aspertame, and afaik this is the reason why they say it is carcinogen. So very much „diet“ stuff and sweeteners has this in, you should be careful.