Question About UPS and Circuit Breakers

Hi all,

I just joined the electronics industry.
There are a lot of things I don't understand.
Firstly, why is UPS using voltage while a circuit breaker is using ampere.
Q1) There is no resistor between them so how does the "change" occur?

Q2) Are busbar and PE bar complete opposite of each other in terms of functions?

Q3) Does any of you know what this abbreviations stand for? IDU as well as PLTS??

Thanks in advance.