Lemmy appears to be very similiar to Mastodon, with decentralised servers to prevent musky spezulence.
Just a bit confused as i've tried Mastodon using the Tusky app and it felt like twitter but decentralised, whilst Lemmy feels like reddit but decentralised but language such as fediverse is shared.
You can't yet, not conveniently anyways. I've seen exactly one post from Mastodon on Lemmy so far, and it was a post testing if it was even possible at all. The formatting was pretty fucked up so it'll probably be awhile until it's an actual feature rather than just a technical possibility.
Mastodon people can post and comment on Lemmy but it makes a mess. I believe they have to @ everyone in a comment chain as well as the OP in order to comment.
A lot of us have separate accounts for different uses. Mastodon is a microblogging platform where one follows users and hashtags. Lemmy is an aggregator and discussion platform where one subscribes to communities (and possibly follows users).
As I understand it, to see posts across services, you just look up and follow accounts and communities using their <@subject@instance.tld> identity. Someone who's more familiar with the process can probably give a better explanation or point you towards a tutorial.