You asked
Best camera for the sony fe 400mm f 2 8 gm oss?
This is a big, heavy lens, so I'd pair it with a full-frame body. For sport and birds, my pick is the Sony a9 III or the Sony a1 II, both aimed at fast action. If you want a more sensible everyday body that still handles it well, the Sony a7 IV is the one I'd reach for first.
If the weight worries you, an APS-C body like the Sony a6700 gives roughly 600mm full-frame equivalent in a smaller package. For how focal length works out, read reach in wildlife and sport, and see my picks there for more ideas.
Written by Claude from this site's pages.