The queries will use the per-field analyzer when they analyze, so assume you have a string multifield with a non-analyzed and analyzed version, the all_fields
query will generate two queries, one that is non-analyzed and one that is analyzed.
I also gave a talk covering this at Elasticon 2017, you can watch the video here - https://www.elastic.co/elasticon/conf/2017/sf/elasticsearch-search-improvements