It depends what you are using.
The usual nail bar adhesives for nail extension which are based around styrene, polyester and polyvinyl resins which are not very good for you at all. An even bigger issue is the MEKP (methyl ethyl ketone peroxide) that is used as the hardener.
These types of MEKP systems will damage and soften nails.
I’m amazed that they haven’t been banned.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7752735/