I imagine it depends upon your phone and it’s support for USB drives, but I can use my kobo Libra and Clara directly connected to my iPad as a drive and drop the epubs and PDFs on it and have it recognize them.

There’s also a website that has released its code (so you can run your own) where I can push things through it via the Internet similar to send to Kindle.

There’s a whole bunch of other reasons that I think the Kobo has the current best eInk device having to do with its interface, but those that are more personal preference related and not the subject of your question. But I’m happy to share if you’d like.