If you are on someones Wi Fi can they access what you are doing
No one can access what you are doing just because you are on the same network. If they don't want you to use their internet, they can block you, but they cannot access your device to see what you are doing.
A good hacker can, or someone with the right equipment. It is possible but not for just anyone.
My mom never lets me turn on the wifi at restaurants because she said those can easily be hacked. I'm not sure about that, but she's my mother. She's usually right.