You can follow a community from any software that speaks ActivityPub. So you can be on Mastodon and post/comment to any Lemmy community.
Alternatively, if you want to use lemmy's interface, you can create your own instance and use it only to subscribe to remote communities. This way, your identity is one place but the commuities is in another.
The unfortunate problem being that anything beyond downloading a program and making an identity is too high of a bar for the average person on the Internet. So as long as this friction of choice exists within AP software, mass adoption is impossible and authoritarian social media like Meta and Reddit will continue to thrive.
Alternatively, if you want to use lemmy's interface, you can create your own instance and use it only to subscribe to remote communities. This way, your identity is one place but the commuities is in another.