Usually Article the is used for Nationalty.
Like, The Chinese, The Japanese, The Indian
Here the sentence is talking about a particular shirt.
We use article the with particular things.
We use a/an for common things (not particular)
Here is the shirt which you gave me yesterday.
Articles "A" and "AN" are used with some expressions.
For example:
In a nutshell, make a noise, give a warning, have a pain, have a great education etc.
S.P. gives vowel sound.
Music is an uncountable noun.
No article before uncountable noun.
Because potato gives vowel sound.
Here police means more than one police man.
He has been working as a teacher since 2014.
These orders of the president may put the lives of two thousand workers in danger.
No article
This is a table.
یہ ایک ٹیبل ہے۔