That’s because Mastodon, PixelFed, Lemmy etc etc are different services based on ActivityPub, they aren’t instances of the same thing; what you want is a software that can communicate with Mastodon, PixelFed and Lemmy
TL;DR: the softwares themselves have to be made in a way to be compatible with eachothers, eg: Lemmy and PieFed works with eachother or you can also use Mastodon to interact with Lemmy (but not viceversa IIRC) too
That’s because Mastodon, PixelFed, Lemmy etc etc are different services based on ActivityPub, they aren’t instances of the same thing; what you want is a software that can communicate with Mastodon, PixelFed and Lemmy
TL;DR: the softwares themselves have to be made in a way to be compatible with eachothers, eg: Lemmy and PieFed works with eachother or you can also use Mastodon to interact with Lemmy (but not viceversa IIRC) too