Ubuntu external SSD EFI setup guide
Ubuntu external SSD EFI setup guide
Ubuntu defaults to booting from the internal drive by default, which is why it appears to install on your internal SSD even though you selected an external one. To change this behavior, you need to adjust the bootloader settings in Windows 10. You can modify the EFI partition configuration or use a boot manager that supports external drives. If you want a reliable backup, consider using a dedicated external drive for your bootloader files instead of relying on the internal SSD.
You check the device settings or system information to confirm it's connected to the internal storage.