Apps can read sim card info. Its country and whether the card is currently in roaming, you can't full them that way

You would need to get your sim card out of your phone and use wifi or buy one of them foreign esim

For vpns I actually found that websites that block vpns for some reasons are worse at detecting commercial vpns than when you deploy your own on vps

---

Also I forgot about other things. When my phone for some unknown reason had its region set as GB, I had British ads in YouTube music

Commercial VPNs publish their exit nodes IPs online. There are services like ipinfo.io which can accurately determine if you are using a known VPN service.