There are a couple ways this could be done.

1. Mirroring content from the other protocol to Nostr relays. This is what has been done with bridge relays like relay.mostr.pub, which mirrors mastodon content to Nostr.

2. Clients would need to be able to access the content from where it is already located on the servers for those forms of social media, and display them to you. OpenVibe, for instance, allows you to access Mastodon, Bluesky, Threads, and Nostr, but you need to have a profile set up on each.

Option 2 really only works if the platform in question allows access to its content so that other apps can display that content. Most of the big platforms are not going to do that.