Usually the websites and apps you use, but not what specific page you visit and it’s content.
If you for example visit https://en.wikipedia.org/wiki/Labor_unions_in_the_United_States they could see that you visited https://en.wikipedia.org/ but nothing more.
This is assuming that the website is encrypted (it starts with https://, not http://), which nowadays luckily most websites are. Otherwise they can see the specific page, it’s content and most likely also all information you input on that page.
Before youtube disabled recommendation for people with watch history turned off, having watch history turned off made it so your recommendations were only based on channels you are subscribed to and possibly videos you’ve liked, commented on, …
There are some categories where I only ever search for the category, then watch a video, but never subscribe to any channel. Those videos were never recommended to me. Meanwhile on my girlfriends pc with watch history turned on, as soon as I watch a single video from a channel she’s not subscribed to similar videos appear all over the front page.