There's a practically official search engine across public instances: https://sepiasearch.org/

I see there is a lot of content posted on lemmy that points to different instances of peertube but I often find it difficult to find it within my instance.

While this is not a solution to the search problem as such, I can highly recommend the Firefox-addon Peertube Companion - it can be set up to automatically re-direct you to your home instance, from any link to a video on another server, that can also be viewed from home. That makes it easy to quickly engage with content from your PeerTube account - like subscribe, like, comment.

As a bonus, it can also re-direct to PeerTube, when it detects a YT video being mirrorred on PT.

Is there an extension or something that automatically archives YouTube videos on peertube?