Mac sharing - I am sharing folders between two macs, but not seeing any files in them

Solution 1:

Is the 'user' accessing the shared folder the same user as the one who owns it?
If it is another user, then it is simpler to share a different folder, one outside the individual User area. The alternative is to add ACLs to the folder, permitting specific access to that other user.

I haven't tested this on Big Sur [which may try to prevent you from even doing this] but on an older OS you'd do it like this…

Select the folder.
Get Info.
Click the lock & authorise.
Click the + icon.
Add another user [this user must already have an account on this Mac] or a group the user is a member of.
Change their permissions to Read & Write.

You end up with this…

You may still have to click the gear icon & 'apply to enclosed items'.

Overall, it is simpler to use the specific Shared area inside the main Users folder, as you don't have to get cavalier with permissions.