Noob Question About Python Web Automation

I've been learning HTML and CSS for about a year, I’m a massive noob at Python and I would like someone to clear some things up for me if possible.

I'm wanting to use a browser extension to automate some work tasks, there's a few good ones that work with JS but I'd like to focus on Python.

I understand that python would require a interpreter to work, I've ran some basic scripts on my PC but I don't understand how you'd execute scrips externally on the web.

Do web browsers have a built in interpreter?

Are there such extensions that exist?

And if so do they have interpreters built in?

Python is a brand new concept to me and I apologise if this is a stupid question. I aim to get the basics down first and then pick up a Raspberry Pi for some project work.