Logon difference in Workgroup and domain

all document and comments focus on central and discrete administration.
I want to ask some detail.

Q1:
in a workgroup, create two users with the same user name(U) and the
same password separately on two PCs(PC1 and PC2).
Create a share folder on PC2 and make it only accessfull to the
user(U).
User(U) on PC1 can access the share folder on PC2.

I can not understand WHY user on PC1 can access PC2's resource as PC2's
local user.
the two users on different PC are definitely two users!
The across-PC access is unreasonable, because if two PCs are
administrated by two local admins, there could be probability that the
two admins create users with same name and password on their PC, it is
not reasonable to allow their resource to be access by a same-name user
on other computer.

Q2

Local administrator VS domain administrator

Who has more power?
What is difference between them when configuring a domain-controlled
COMPUTER or service?

.

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