Definitely not a stupid question! Networking infrastructure is complex. I’ve been working in IT for years and still find myself scratching my head at times going, “Wait, how does the OSI model work again?”
Connecting to a VPN on your phone while using mobile data basically means the cell phone tower handling your data only sees encrypted data. Whoever your VPN provider is will see your traffic instead of the cell tower.
However, in modern times it’s fair to be wary of backdoors and exploits that can compromise your device and render the VPN encryption moot. There’s not much that regular people can really do to mitigate that possibility other than not use a phone.
If you’re interested in learning more networking fundamentals, I’d recommend starting with the OSI model and its layers.
A handy mnemonic I whipped up with ChatGPT last year for better remembering the order of the layers:
Precise Data Navigation Takes Some Planning Ahead
https://www.youtube.com/watch?v=fkNqNYkwhlg