I using using a 4G connection with a daily limit of 3GB.
How do I stream Youtube using MPV specifically in 360p ?

I know of no way that one can dictate to Youtube the size of a stream.

A web search for "youtube manage stream" turned up many articles for those who create streams, but none for those who download them.

If I am in error, as my old boss used to say, I am certain that someone here will correct me.

Not everybody knows that that entails using youtube-dl, you should've mentioned it.

Reading you can pass options to youtube-dl with "--ytdl-raw-options".

Or you can put them in ~/.config/youtube-dl/config (read man youtube-dl for more info).
Restricting video resolution to max. 360px height would be done like so:
It is possible that youtube-dl does not offer all videos in this low resolution anymore.

youtube-dl offers only what the streaming service offers. --list-formats to see what's available. and --format ###,###,### for whatever ones you would like to acquire. When using a browser there's a setting gear to select a lower resolution. Which has oddly been adjusting to 1080p even though I set it to 360p manually. Depending on the best capability of your network speeds. Which for my current ISP, will surge a 1080p stream and then buffer a lot. It might do a 720p stream if it's the ONLY thing using the network and it's off peak hours. But 360p is a nice spot where others and other things can still use the internet while youtube plays a video. And even that will still buffer at times.