Creating a fusion clip is like doing a PreComp in Ae. Once created, the clip is treated like footage. It has a given length. Inside that PreComp is the piece of footage that you made the PreComp from. It's what defined the length of the PreComp. You drag the media in node up to a viewer. After that you go to town adding nodes.
Once you're in the PreComp you have to add more footage. To do that is where the analogies fall apart.
You need to add a Loader node to bring in additional footage. Honestly, there's too much just to write up like this. Go on YouTube and watch a few tutorials on Fusion to see how the work flow goes. There's Loaders and Merge nodes that are going to frustrate you at first but I promise it gets really cool and blows Ae out of the water once you get in the groove. The only thing that I have to make work better are short cut keys for position, rotation, etc.
Ae is great for slapping things together and reordering which things come first, second, third and so forth. To do the same you need to learn how Fusion works. It's foremost a compositor which means you work on one plate at a time. All your footage has to be the same length. Even text that appears for the first three seconds of a ten second clip has to be ten seconds long. You have to do something like turn down the Blend to zero for the rest of the time. Or freeze the last frame on a piece of footage for the rest of the time and then turn down the Blend to zero. It's super unintuitive and feels sloppy for Ae users.
There is a way to do it with a node called Multi Merge which I posted about earlier this week when somebody asked about managing scenes but to use that you have to install Reactor. Google for Black magic reactor and you should find it quickly.
You can set up Fusion to work like Ae but you have to get some preferences in order first and convert some terminology in your head. Good luck.