The snap declaration is a signed assertion (document) that corresponds to your snap. You can view it by doing “snap download foo”; that will download both the .snap file for the snap and a .assert file (feel free to look at it).
The ‘base declaration’ is something encoded in snapd that declares how interfaces can be installed, connected or auto-connected (you can see it with sudo snap debug get-base-declaration). The ‘snap declaration’, among other things, may have overrides to the base declaration for snapd to consider when evaluating things like auto-connection. The snap declaration is something that the store maintains and signs (snapd will verify the signature as needed) and reviewers are able to modify the portions of the snap declaration that relate to overriding the base declaration and setting aliases. In this manner, snaps just plugs and slots as desired and reviewers can update the snap declaration for a snap as per our processes.
Correct, just follow the processes you’ve always used to plugs and slots things and continue to make requests for auto-connection, etc in the forum like always. Snaps do not need to be updated. The information in the aforementioned link is primarily of interest to snapd developers and store reviewers, yes.