Can My ISP See What I Do When I Use a VPN?
A VPN changes what your internet provider can directly observe, but it does not remove the provider from the connection path. Understanding the boundary is more useful than treating a VPN as magic.
What your ISP can still observe
Your internet provider still carries the connection between your device and the VPN server. It can generally observe that your connection is exchanging data with an IP address used by the VPN service, along with timing and traffic volume. The exact visibility depends on the connection and protocol.
What encryption changes
With a properly configured encrypted VPN tunnel, the ISP should not be able to directly read the contents of traffic carried inside that tunnel or simply inspect the final websites from ordinary encrypted tunnel traffic. The VPN provider, however, becomes an important party in the connection path.
What a VPN cannot solve
Signing into an account identifies you to that service regardless of your IP. Cookies, browser behavior, malware, account activity and information you voluntarily provide are separate issues. Privacy works best as layers rather than a single switch.