When the bathtub in my upstairs bathroom emties, it sometimes overflows the toilet in my downstairs bathroom, why is this? The water is definitely the water from the tub as it is warm and bubbly

Clogged sewer drain pipe is the culprit. There is a clog further down the line and this comes out when the water from the bathtub backs up with no where to go except into your toilet. This clog needs to be found otherwise you could do a lot of water damage very quickly with this type of arrangement.

Is there no tub or shower in this lower bathroom? The drain line isn't clear just like twink says and the tub many be the next fixture downstream from the upstairs tub.

A partial blockage just below the downstairs toilet.
Tubs get hair clogs very easily so try the usual drain cleaners, possibly a few times, and make sure you use a hair trap over the drain (hair gets through anyway though, just more slowly).

Thank you for this help, I never thought of a hair trap and will definitely try it! (weird thing? Im a hairstylist! LOL guess Im no plumber!! ) Thanks again

Thanks for your help!

Now it is making more sense. Thank you

I doubt that this will turn out to be hair. If only the tub was clogged I'm with hair but this is a main drain line that both toilet and tub drain into. This will be a more significant clog and will require rodding. If you find no cleans that can be opened to let you rod then remove the overflow plate from the tub and rod down from there. You may need to run 25' or more snake. If you can't reach it then the toilet may need to come up since you can't rod a drain through the toilet

Joy hit the problem in her first sentence and the lost it in the second. You do pave a partial blockage downstream of the toilet but it won't be hair from hair from the tub.
Since the clog is in the basement I would snake from a basement stack cleanout or if you don't have one pull the toilet and snake from there. Good luck, Tom