You can view the source code of all extensions installed in Chrome. The folder names are gibberish, but if you look at Date Created you can usually figure out what extension corresponds to which folder.

Windows: C:\Users\<user>\AppData\Local\Google\Chrome\User Data\<Name>\Extensions

Mac: ~/Library/Application Support/Google/Chrome/Default/Extensions

Linux: ~/.config/google-chrome/Default/Extensions/

The extension needs access to the DOM when the page is loaded to find and replace words.

The extension works locally on the client. It does not send your data anywhere or talk to any third-party servers.

Any reason you replace, rather than insert alongside?

I like the sporadic messages, but placing them roughly a minute of reading apart as [posture] reminders would make me more likely to use it ;p

No reason. I like the idea of randomly inserting them.