Using a VPN makes your traffic travel through the VPN server to get encrypted before reaching the destination.
Using Tor basically does this 3 times, but it’s decentralized so it goes through multiple different random relays before reaching the destination. And it changes which relays you’re using every 10 minutes.
When using a VPN you’re basically relying on your VPN service giving it their all when it comes to protecting your privacy, and also on them not bending over to the government if it wants to monitor you. Which you won’t get with a lot of VPNs (especially not free VPNs).
Since Tor is decentralized and changes your connections frequently, it’s virtually impossible to monitor someone using Tor. The chance that all 3 relays your traffic travels through are controlled by people coordinating to get you are slim in the first place, without even considering the relays changing.
You can also use both Tor and a VPN at once, but to do so properly is a lot more convoluted than just turning on your VPN and using Tor at the same time.