It may surprise you but people are still building medieval castle in France: https://www.guedelon.fr/en/ (youtube has numerous video from them showing medieval technics of building) And France keep some of the building tradition through something called "compagnonnage" https://fr.wikipedia.org/wiki/Compagnonnage (sry but there no en page for this article).

Something close in en: https://en.wikipedia.org/wiki/Compagnons_du_Devoir

The answer is simple, there's an endless amount of churches and castles to maintain in France, so many of them that it's actually impossible to maintain them all. As long as the state cares about maintainance, there's going to be jobs for it.

I know that the bells were made in the Netherlands- which has a long history with church bells.

If you're in the US, you can apply to attend the American College of the Building Arts.

https://acba.edu/aboutacba

They were established in Charleston SC after Hurricane Hugo when it was discovered that the US did not have enough craftspeople to repair all the historic homes that had been damaged. You can get an accredited Bachelors degree in architectural ironwork, plastering, timber framing, and stone carving.