Time Machine is full?

Trying to work out what to do about it. I’m not buying another bloody ext hard drive.

Can I delete everything up until 2018—or something like that—to get space back?I mean, I’m sure there’s nothing different version-wise, pre 2018, that I’d need to recover. Whatever is on the Mac's internal hard drive is what I need to keep.

But perhaps I'm misunderstanding what Time Machine does. I assume this:

I think it does a big backup the first time, of everything, then after that, only backs-up stuff that’s changed and is new/added—I THINK. It’d make sense to me.

So if I delete everything from 2014 will I lose most of my files, or is it intelligent enough to keep all the stuff from 2014 that never changed?

Thanks

John