Depending on your experience with C, you can go straight to the Fortran standard and review it first: https://j3-fortran.org/doc/year/18/18-007r1.pdf
You can then review webpages such as the following for some additional detail:
https://splichal.eu/scripts/sphinx/gfortran/_build/html/interoperability-with-c.html
You can also consider the following book reference: Modern Fortran Explained (MFE) including Fortran 2018
https://oxford.universitypressscholarship.com/view/10.1093/oso/9780198811893.001.0001/oso-9780198811893
And do a search on this forum for “interoperability” and you will see a few hits (I’ve posted several) with targeted snippets such as the one upthread.