There are multiple ways to make a search on PeerTube. The administrator can enable or disable each one.
In this mode, the search will only display results known by your instance (so videos and channels of instances followed by your current instance).
Has to be enabled by your admin
In this mode, your instance will use an external global index to make a search. The results are not scoped to videos and channels known by your instance. For example, you can search videos of instances that are not followed by your current instance.
Could be disabled by your admin
You can search a specific remote video that may not be federated with your instance using its URL (for example: https://peertube.cpy.re/videos/watch/da2b08d4-a242-4170-b32a-4ec8cbdca701).
You can also search a specific remote channel using its URL or the channel handle (for example: https://peertube.cpy.re/accounts/chocobozzz or