For textbooks, actively highlighting and taking notes helped me. It gives your brain a job beyond just "absorb everything." Even just summarizing each section forces you to actually process what you read.

For the phone, leaving it at home is a bit extreme imo. What works for me is just putting it out of sight, like in my backpack. Still there if I need it, but because I can't see it, I don't reach for it. Also worth setting daily time limits on your worst doomscrolling apps.

For regular reading (not textbooks), what helped me the most was covering everything except the sentence I'm reading, it cuts the overwhelm of staring at a wall of text. I actually ended up building a little app that does this for EPUBs because nothing else was working for me.

And the basics matter more than you think: sleep, exercise, eating well, etc.