On my phone, I use "Notepad - simple notes" aka "Notepad Free" by atomczak. According to the app store, it has ads, and some people have reported seeing them, but in my several years of using it for free, I've never seen one. It's a basic notepad which is perfect for my needs
When I used a Samsung Galaxy Note, I would use S Note which was similar, though it had the benefit of using the stylus, which I kind of miss having
On my computer, I'll either use Sublime Text or vim, depending on which context I'm working in