How much of what I see on social media is algorithm driven and specifically being shown to me versus normal stuff everyone sees?

The algorithm shows you more of what you engage with. It doesn't matter what that something is.

Engagement can be liking, sharing, receiving messages from others, but also something as simple as pausing on a reel for longer than usual.

There is no "normal" stuff that everyone sees.

This behavior is precisely why social media feeds create bubbles of interest or opinion where people become completely immersed in one thing (such as ADHD), or at the extreme end become politically or socially radicalized.

Put simply, you have to actively try to find other content that broadens what you see.