Fortran under Big Sur
First let me say I’m not a big Fortran user but any blog posts about Fortran always seem to be very popular, and the Fortran on a Mac page is one of the most popular pages.
I've been sent these details of fortran compilers that might be of interest.
NAG Fortran compiler Fortran compiler for Apple Silicon Macs now available to download.
Absoft Absoft Pro Fortran 2021 For macOS and OS X Fully compatible with macOS Big Sur (11.0).
Computation Tools :: C/Fortran On the HPC Mac OSX page, Compiled using source code from the GNU servers.