Q: Is my ISP spying on me? If yes, how can I prevent them?

A: It is almost certain that they are.

Not like they have a person on the payroll to spy on your activities specifically, but virtually all ISP’s (ALL in the USA) log your activity, and most (if not all) sell this to marketing firms, so that the marketing firm can better target advertising to you. In the US it is a requirement so that government authorities can monitor the activities of criminals, suspected criminals, ex-spouses, current lovers, and anyone else they care to harass.

What to do about it? Get a quality (not a free or inexpensive) VPN service with DNS Leak Protection, and then use it full time while on the internet. My current recommendation is NordVPN.