3) The _Makinom game object (in the DontDestroyOnLoad part of the scene) has informations on all running machines - e.g. pause the play mode and check what else is running.
If it's coming from a scene change you'd probably notice that the scene changed before :D
4) Usually via adding actors to your schematic (in the Settings node).
There are different types available, e.g. you can search for objects in the scene (by name, tag, etc.), have a machine component select one in the scene setup, etc.
The Game Object > Component nodes in schematics have a lot of component-related stuff, e.g. the Enable Component node to enable/disable components.
5) Use is pretty simple, you add machines (i.e. schematics with machine/starting object) to a stack using the Add Machine To Stack node. Afterwards, you can execute them in the order they're added via the Start Machine Stack node. The machines will perform one by one until the stack is done.
You can e.g. use this to have an NPC during a cutscene do a series of things while the cutscene schematic continues with other stuff.
6) Start menu and menu screens are separate things.
Start menu is just for the game start, i.e. new game, load, etc..
Menu screens are the in-game menus to handle inventory, equipment, group, etc.
They're not used together, i.e. menu screens only work in a running game, while the start menu is usually only used before starting the game.
There's not really a built-in feature for such a scrolling text credits display, but I'm sure there are a bunch of 3rd party assets out there to set up something like that. Or just set up a canvas in your scene with such a scrolling text that you can turn on/off e.g. via schematics.
7) This can be set up per item type and individual items can also override this.
The Keep Open setting in the item's/item type's Inventory Settings handle this. If enabled, combatant selections in menu screens will keep open to allow using them again and again.
Abilities also have that setting (per-ability setup, i.e. no type-related setup).