It all just depends on whether it makes sense or not, and also whether the developers of each software coordinated to make it translate.
Like you point out, text posts don't exist on a video platform (at least if you're not counting youtube community posts), so peertube devs didn't write any code to receive or display them.
How you get stuff to show up also depends on the platform, but I believe most will do it if you search the account handle of the account on the other service - so you can follow a peertube account from mastodon by searching @account@example.com
and then it starts showing newly posted videos in your feed.
Since all the translations are decided by the devs of each software, some of it is a little funky - like IIRC lemmy communities are displayed in Mastodon as an account that you can follow, and each post to the community is a post on that account (or maybe it's boosted by that account, I can't remember), so if you follow the "community account" from Mastodon, you will start seeing new posts in your Mastodon feed, and can reply to the comments by replying to the post. You can also post to the community from Mastodon by mentioning the community at the beginning of a Mastodon post, and I think it will boost it.
It's kinda weird IMO - I get the feeling a lot of posts from Mastodon in Lemmy communities might be made by accident because it often looks like the Mastodon user was just trying to tag an organization (like tagging @Plex or @Netflix to complain about their software or something) and then saw the account suggestion from Lemmy and didn't realize they would be posting to a community instead of tagging an individual.