@asklemmy So, is this how I post to a Lemmy group if I'm only on Mastodon and haven't even made a Lemmy profile?...

You are in this thread!

You can’t see all of mastodon from lemmy though. Only when a mastodon user interacts with lemmy.

This can be done in two ways.

1. Replying to a lemmy post or comment (they can upvote too)
2. Creating a new post to a lemmy community, which is what this post is.

But because mastodon (and microblogging generally as there are other fediverse microblogging performs) is based on following people, and you can’t follow people on lemmy, all of the interpersonal mastodon stuff, though public, is not visible to lemmy.