As far as I know they go to the moderators of the community that the post was in. Admins have some level of oversight but for the most part individual communities moderate themselves.

For example, if your account lives on lemmy.ml, but you’re browsing content from gaming@beehaw.org, when you report a comment it will go to the moderators of gaming@beehaw.org to take action. Admins who oversee all of beehaw.org can see a log of moderator actions across the whole instance, but for the most part they don’t participate in moderation except in very small communities.

It’s also important to remember that some communities were stood up very quickly and in some cases the people who started them and are the only mod might have stepped away and are no longer checking on reports. In this case it would be good to reach out to a site admin to take action and appoint a new mod of the community.

Hopefully that makes sense!