As long as you have the itemId, there is no need to specify the path. For example, this is how to get a specific item within my ODFB site collection:
GET https://graph.microsoft.com/v1.0/me/drive/items/01BATTRZNBUNZS4552UNF23DBJY5IMLUJH
For SPO site collections it's a bit more complicated, as you need to provide the siteId and listId first, but items within the library can be enumerated by simple incremental numbers. Here's an example:
GET https://graph.microsoft.com/v1.0/sites/tenant.sharepoint.com,33824aad-cb6e-464f-8a84-ccc371607728,4cc0fca1-f010-4e3f-b8b8-136f1d6fa200/lists/3fdbd1c5-fc3b-4b77-9dc9-625d61b859b6/items/55
where I'm getting the 55th item in the Shared Documents library of the default site collection.