How did people use ed?

ed was the standard Unix editor, and is present on all POSIX certified systems (though it's not installed by default in Debian, FWIW).

It's a line editor (meaning, you can't see all the text at a time. You query line numbers and it spits it back, but it came before vi[1], where you can use hjkl to navigate up and down the screen).

How was it used? I find it hard to believe that people memorized their code well enough that they could remember what was going on in line 5 when working on line 20[2]?

Did people write code on a paper and then type it in (sort of like in the punch-card days), editing the paper and then modifying code through ed?

[1]. Small point, ed gave birth to ex, and vi was ex's "visual mode", and ex's legacy is still visible in modern vim/neovim (when you type :COMMAND, like :w, that's vi running an ex command), so ed's legacy is still around to this day.

[2]. Programs weren't too long in the 70s. There just wasn't enough RAM for them. But even a short 20 line program is still too complicated to fully keep in my brain, line by line.