in what year vs. in which year

English as a Second Language English Vocabulary, Grammar and Idioms
1

Why we use “In what year” instead of “In which year”? What’s the difference between them?

2

In what year - out of all the possible years in existence.

In which year - from a series or selection of years.

For example;

In what year did Queen Victoria commence her reign?

In which year between 1830 and 1940 did Queen Victoria commence her reign?