Hi, so this is quite old but let me guide you if you need to use them this thread was with 5.0 I believe, If you use the vehicles templates find the SportsCar_Pawn on that blueprint on the movement component you can search to filter with the keyword Mechanical Setup, that category has the default configurations for the template and those are the veriables that specify how the car works.
And those variables are used by the Set Throttle Input node on the event graph
No, you an define Thrusters on the same movement components search with the keyword thruster setup and on those variables you can define the configurations that you need.
If you define any value on the thrusters configurations the movement component will use those definitions to use the thrusters, now the only problem is that you dont have a dedicated method to set the input on those thrusters and you have to use the same method that you use for the Mechanical Setup definition, internally that function process the input based on the mechanical setup configurations and then goes and process the input based on the thrusters configurations, if any of those are not defines the movement result for that configuration is 0 if both are defined your movement result is going to be the sum of both inputs (Thrusters movement + Engine Movement).
There is also a reddit thread with the discussion before I reached this information
Also, I think this is an already know think but if you try to make a multiplayer game with the vehicle movement component you need to keep in mind that that movement component does not handle the client side prediction as the Character Movement Component does so if you have that feature you will need to implement that by yourself.
Let me know if something is not clear so I can explain a little bit more or if something does not make sense because english is not my native language.