How to know the equivalent models between NVIDIA and AMD graphics cards?

I never really understood much about this. But would a 9070 be equivalent to a 5070, for example? Or should you not rely on the numbers?

Another question I have is that I always see people buying NVIDIA. It’s rare to see someone getting AMD. Why? Is AMD more expensive? Which one would be better between the two?

I’ve never had AMD because I’ve always bought my gaming PCs prebuilt, but based on how good AMD processors are, I would buy a graphics card from them. Also, having the graphics card and the processor from the same brand, doesn’t that help with something?