I think the default is to apply equal weight to each term in the box (not really an "AND", but not really an "OR", either).
This document explains it in more detail:
Thank for your Answer.
I don't find in this documentation anything for searching two or more terms without Operator respectively without quotation marks.
The result of a search inculde results with only the first term and results of only the second term. That's in my Opinion an OR query.
EDIT: @ Matthew
Sorry, you are right. As a first all results are displayed with two terms.
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