Hi, I'm trying to wrap my head around LOPs.
I've used the 'layout' node with the 'point instancer' method to place my assets.
Later, I extracted specific point instances with the "Extract Instances" node.
My goal was to modify these instances using physics, since point instances don't work with physics...
Now I'm looking for a way to convert these edited instances back to point instances.
I'm an idiot, please help me.

There is no simple way to do this. It depends a great deal on whether you are just updating the transforms of the instances, or are fracturing certain instances. In the latter case, there's not much point in trying to shove fractured instances back into a point instancer because presumably each fracturing and each piece will be unique. So I'll assume you are talking about the former case, where I'm not sure why you wouldn't be able to do this with point instancer instances (though at the moment it may take some scripting to transfer the xforms calculated in SOPs/DOPs back into the point instancer transform attributes...

As Mark said, there is no easy way to turn extracted point instances back into point instances. If you are using physics to only move them around however, you could use the Modify Point Instances LOP in a couple of different ways to get those edits back onto the original point instancer.

In this example hipfile, the first modify point instances LOP ("modifypointinstances_physics_inside") is used to run physics on the instances directly - dive inside the node and there's an example of turning them into RBDs to move the point instances around.

The second modify point instances LOP ("modifypointinstances_match_edits") is used to bring back changes done to extracted point instances by copying their packed full transform in SOPs onto the instance representations inside the modify point instances LOP.