Not video, but I will try to explain it in a simple way:
Lemmy instances are individual websites that run the Lemmy software. These instances are operated by different people or organizations and can have their own unique topics, rules, and communities.
Lemmy instances federate using the ActivityPub protocol. This protocol allows different servers, or instances, to communicate and share data such as posts, comments, and community interactions.
Different instances use the same software, but are managed by different admins.