There are a few I found in Github here: https://github.com/topics/lemmy-bot. This one looks like it's a pretty generic bot: https://github.com/SleeplessOne1917/lemmy-bot

There is an API for Lemmy, though the easiest way to make a bot would be to use one of the official libraries to talk to Lemmy: https://github.com/LemmyNet/lemmy#libraries

Not aware of any guides, but the most complete resource seems to be the lemmy-js-client repo, specifically the docs for LemmyWebsocket.

I want to, but I am trying to find a way to make a private backup of my saved comments/threads before I do. I have a lot of useful stuff that I saved over the last 12 years that I really don't want to lose. Some have suggested making a GDPR request, but since I am not in the EU Reddit technically doesn't have to comply. Automated tools I have found only purge my data with no way to make a backup first.

Lemmy is divided into a backend API and a front-end user interface. You can either make a bot that is specific to Lemmy by programmatically using the backend API, or hypothetically make an agnostic bot that works over the ActivityPub interface.