I have a link to some instagram post/reel (e.g. https://www.instagram.com/reel/C0w1dkGr4fi/). I need it's ig_media_id to query the Instagram Graph API, how am I supposed to obtain this, it doesn't show up in the docs as far as I'm aware.
You can use the /{instagram-business-account-id}/media endpoint with the relevant fields to get the ig_media_id. Use the following:
graph.facebook.com/{instagram-business-account-id}/media?fields=id, ig_id, permalink
You would need to map your reel url with the permalinks presented in the responses and then pick up the respective ig_id.
Pro Tip: You can use the Facebook Graph API explorer to test the endpoints quickly: