I am looking for someone who has the experience and knowledge how the
retirement amount is calculated for someone who has worked let say 25
years in france and 15 years in the US as there is an agreement
between these 2 countries to pool everything together

I don't have any knowledge regarding France and the US but based on
experience with other countries, I would say 'Unlikely'.

However, I would guess that you should qualify for benefits from both
countries, if you are so entitled.

If you had worked for a multi-national company in different locations
then such a company may have taken steps to ensure some consolidated
retirement benefits for you.

I hope that you have kept all the paperwork!

Good luck.