I just received the above question on LinkedIn and like to share the recommended resources to all:
(I skip the link since I am not sure if the asking person likes to be published)
There is a great blog post from Lauren at the MongoDB developer hub:
For further reading and details:
I covered the subject in my latest (as of June 2021) MongoDB User Groups for the DACH region.
Just move straight to 1:08:00 were we take care of the case insensitive queries
Chapter 3 is about indexes, I am not 100% sure how deep case insenitive indexes are covered.
Hope this helps