Well, essentially, all of the traditional text-only clients do (such as Mutt, which is what I use), I think Thunderbird does as well, beyond that, no clue, but there probably are others.
Traditionally, if you chose to reply to an email, the mail client would put the mail you were responding to into your editor, with > marks in front of every line. The only socially acceptable way[1] to write your answer was to insert your responses in between those quoted lines, without > marks, and to delete everything of the original mail that was irrelevant to establish the context for the reply.
That's how easy it is.
Usually, a good mail client would also color quoted lines so that it's easy to see and read only the response lines in a received email, so you'd only have to read the quoted text if you weren't sure about the context.
Clueless developers and users around the turn of the millenium then somehow didn't understand the purpose of providing you with a quoted version of the email you replied to, and started putting the reply above the quoted mail, thus ending up sending a copy of the mail they just received back to the person they received it from ... and nobody seemed to notice how idiotic an idea that actually is, and so it became sortof the new norm in large parts of the internet to attach large blobs of completely useless and often close to unreadable text to every mail, up to the point where some people even invented justifications for the behaviour that mostly tend to describe how this "feature" allows them to work around the lack of proper thread handling in their mail clients.
Obviously, easy quoting traditionally relies on plaintext only mails with fixed (short) line length, but format=flowed encoding has since been invented (the original RFC is from 1999) to accomodate screens and windows of variable width while maintaining backwards compatibility with old clients.
[1] https://www.netmeister.org/news/learn2quote2.html