The effects of switching between graphics cards

I have a Dell XPS 15 9570 with Ubuntu 18.04 on it and the following graphics cards:

• Intel® UHD Graphics 630 (Coffeelake 3x8 GT2)
• NVIDIA® GeForce® GTX 1050Ti 4GB GDDR5

The first is integrated, and the latter is dedicated. Integrated means that it uses the CPU and System RAM for graphics processing.

I know how to force switching between these cards, e.g. to select the Intel GPU:

sudo prime-select intel

I am trying to understand how Ubuntu makes use of the cards available.

More precisely:

1. Is there a default GPU? If so, how can I check which one is the default one? How can I check which one is being used?

2. What does it mean to prime-select a graphics card? Does it mean that it will always force the GPU computations to be done on the selected graphics card?

3. An answer here says that Ubuntu does not switch between the cards to balance power/performance. Does this mean that the same task (e.g. running a game) cannot be done on both GPUs at the same time, and there cannot be any switching between cards during the processing of the same task, but for different tasks Ubuntu may decide to run them on different GPUs? Or does that mean that the System will always use only one card (the default or prime-selected one)?

Thank you!