Pulling content from other nodes into a Panel Node
When you add an Object to your Panel Pane, you will find in the left column the option to add an "Existing node", choosing this, you can add a node id of the content you want to add.

If you want to dynamically provide different nodes provided by the user input on the node form using a node reference field, you need to add a context to the panel by using this reference field and then the referenced node will also be available.