How is text not easy to modify on your phone? If anything this is a problem with phones being terrible computers and not text files. But I don't see how anything I said about how I do it on a desktop computer does not apply also to phones.

Not only that - how do you do structure? I'm especially thinking about trees here, which is how most concepts seem to be presented the best.

I store my text notes on Dropbox, using various subfolder layers. The android app allows both creation and editing of .txt files.

I self host Seafile. It has a simple and markdown friendly editor in its mobile app.