The explanation given here are very good. I just want to had add that theoretically, any Lemmy instance can communicate with another instance from another fediverse software but in practice, developers haven’t had the chance yet to make sure every feature is compatible with the each every other software.
If you want to try sending message across the different instances and softwares of the fediverse without spamming or flooding people discussion spaces there is a dedicated community: !testfediverse@jlai.lu