How do I get my iPhone SE to work overseas?

Talk to your carrier about their roaming policies if any. If their roaming price is too high or they don't support international roaming, you can look into getting a SIM and a prepaid plan from a French cell carrier.

You may find this worthwhile reading:

https://esimusa.com/blog/ios-travel-internationally/

and others here may be able to offer specific advice.

Regards.

And if you do go with a different carrier, make sure your phone is unlocked from your current carrier.