The short version: It doesn’t matter if you use Lemmy, Mastodon, Friendica or any other platform to participate in the Fediverse.
The Fediverse is an open stream of data (writing, images, videos, geodata, etc.) and all projects in the Fediverse can receive this data stream.
There is a certain degree of specialisation in order to better meet certain challenges. For example, you can easily share long videos with Peertube. Pixelfed specialises in the sharing of images and Lemmy in the aggregation of links.
There are also sites such as Mastodon, which specialises in microblogs, or Friendica, with which you can write long content and structure the content with Markdown or BBCode.
This content is created with Friendica, for example. So I don’t use Lemmy. Nevertheless, they can see the content and we can interact with each other. This is because our software, which we have used to log in, is connected to each other via the same data stream. Only our front end looks different.
But this also means that with your Lemmy account, you can receive all the content if it is available in the data stream. Enter a few hashtags that you would like to be displayed. When posts with this hashtag are published, you should be able to find this content in your personal timeline.
Thanks for the hint. I do know Lemmy, but not in the depth that is required ;)