Car chat - need a new tyre

[WoodyCoppicePlantationAlmaMater]

Passed the mot but with an advisory on one of the tyres as although the tread is OK, the rubber around the rim is perishing (it's an old tyre).

My question is, do I replace just this one or should I replace the opposite one also?

Tyre in question is the rear drivers side - so do I also need to replace rear passengers side also? Or is it OK to replace just the perishing one?

I know nothing about cars.

Fine to replace just one, I would do same brand and type
If the other was v close then I would do both but there's no point getting rid of a decent tyre

Best practice to replace both on the same axle at the same time.

How old is the rear passenger side one at how much tread is on it?
If you look on the sidewall of the tyre there is the word DOT and 4 numbers after it. This tells you when it was made e.g.1120 means the 11th week of 2020. The drivers side one must be really old if it's perishing.

It depends on how the opposite tyre is doing. If it has loads of tread on it, no I wouldn't replace it. If it's MOT worthy but measuring close to 2mm, I'd get both. Might as well while you're at it.

I replaced them one at a time as needed. Until I took my car to have one replaced, and killed time by looking round the dealership opposite the garage. It was the most expensive tyre ever as I brought a new car!