Every post has an “activity pub id” (ap_id in the api) which is a URL to the post on its home instance. Every Lemmy instance can get a link to a post by its activity pub id.

The flow is:

• parse the URL and get the numerical ID
• call an api endpoint with post details (on the instance the link points to), which includes its activity pub id
• call an api endpoint on your target instance to resolve the activity pub id to a local post (which includes its local ID)
• construct a new URL in the form of https://[targetInstance]/post/[localID]