Android/Windows/Mac/Linux: Wikipedia, for all its flaws, is an unparalleled wealth of information. It’s also entirely unavailable without an internet connection — unless you use Kiwix.
Kiwix is a Wikipedia client that allows you to download the entire library of articles to your device locally for use offline. The entire English collection is about 18GB, which might be a bit much for your phone. However, there are smaller chunks (such as one without pictures) you can download as well.