> I have never encountered a single French person who uses anything but their first name as their "Prénom usuel".

Most people who chooses to not use their first name as their "prénom usuel" wouldn't disclose to people their aren't close with that they don't use their first name.

You wouldn't know unless they told you their full name. Witch, let's be honest, rarely happens outside of administrative work.

It's totally possible that you encountered multiple peoples using a "prénom usuel" witch is not their first name without knowing it wasn't their first name.

No, your "prénom" is your "prénom usuel" witch can be chosen between any of your "prénoms". It's the "prénom" you use in everyday life et nothing force it to be your "premier prénom ". It usually is the "premier prénom" but a lot of people uses one of their other "prénom".

Not everyone but a lot of people have multiple "firstnames". It depends on family traditions etc. I personally have 3 names before my family name. My sister have 4.

Our parents gave us our names to honor people from the family. My second name is the firstname of my great grandfather and my third name is the firstname of one of my great uncles who died shortly before my birth.

Yes but here the precision is important because the data is specifically on the "premier prénom".

"premier prénom" is not redundant as you can have multiple "prénoms". Your second name is your "deuxième prénom" and your third name is your "troisième prénom" etc. With your last name usually being one or two family names.