How does elasticsearch under the hood?

I have deployed and configured elasticsearch on server, installed elasticpress on the website, did the sync and I can see over 5000 posts are indexed, IOW, things seems to work fine as per documentation, that said, I am not sure I understand how this works under the hood, we have custom ajax search that goes into custom post types, custom taxonomies, and custom input fields within each custom post types, my questions are as follows:

1) Does elasticpress index custom fields, custom post types,and custom taxonomies? based on my test, it doesn’t index custom fields, could anyone please confirm? If the answer is yes and we can index custom fields, how can that be done?

2) Does elasticpress alter default search queries automatically? do I need to edit search queries to make use of elasticpress?

3) I tried to see if Elasticspress can work with plugin such as relevanssi, but it seems it’s either this or that, can anyone please confirm?

Appreciate all the help, thank you so much!