Extend a comp indefinitely by stretching its layer

Hello

I received a source file from a colleague with a very strange comp (probably created by a plugin) - when placed inside another comp, I'm able to extend its duration by draging its edge indefinetly, without any limitation.

Additionally, there is an animation set to the end of the comp, which moves correspodingly to the new end of it when I extend its length.

I hope I explained it well enough

My question is - how the heck was this created, and how can I create it myself easily?

Thank you