A quick look at the https://join-lemmy.org/docs/administration/federation_getting_started.html does not answer that question. Though at least ChatGPT has an opinion (which may be misleading) 😀

the probability of it (the post) being distributed on multiple instances depends on factors such as the popularity of the post, the number of upvotes and comments it receives, and how widely it is shared across different instances. Posts that generate a lot of engagement and discussion are more likely to be distributed on multiple instances, as they are more likely to be shared and reposted by users. Additionally, posts that are deemed relevant or controversial may also be more likely to be spread across multiple instances.

I am unsure what it means by shared (i assume just posting a link is not enough).

If that is true that reposted (often called cross-posted) post makes a fully "featured" copy/backup on a different Lemmy instance, is there anything else that does it? Thank you