Focus on making posts you want to discuss or would want to see in communities that interest you and you want to see active.
While I’d argue it’s better to shake off the platform thinking, the simple way to put it would be that you simply refer to the site you’re using, feddit.org, when mentioning it to others. The umbrella term for these connected sites is either fediverse or the open social web, whichever you prefer. Each site like this connects with one another, but given formatting differences (Pixelfed is more image-focused, Mastodon is microblogging), posts shared between them try to display in ways fitting one another’s format which can sometimes look rough.
You may see content from these other sites when browsing the main feed/front page set to All, which displays content from elsewhere that others on your site (feddit.org) have subscribed to.
No, sorry. To put it in your terms Lemmy would be a “platform” like how you describe Mastodon/Pixelfed.
The reason I suggested mentioning feddit.org instead is because it’s what you’re using and where someone else could sign up and join easily.
Mentioning Lemmy (or Mastodon/Pixelfed) doesn’t tell people any site to sign up on, just what tech they’re built with.