I use a fountain pen with a thin A6 paper notebook, one private and one for work. I start a new page for every new context, and put off-topic notes (random thoughts, names of people) into the right margin of the current page. Things I want to look up later get annotated on the left margin. My work notebook also has an error log on the last pages.
Realistically, most on-topic notes are write-only, but the process of writing them helps me to remember the content, and on occasion I actually refer back to them.
I like this system because I neither need to carry a laptop or keyboard, nor need to type on a phone. My notes also don't clutter that much – on a laptop, they tend to stay on the desktop "so that I can easily go through it later", which of course never happens.
A disadvantage of my current system is that the notes are not searchable, and do not have a backup. I considered updating to a reMarkable, but so far found the price tag too high for my purposes.