What is the difference between bridge table and helper table in data warehouse's context?
Bridge tables are the official name according to Dimensional Modeling as defined by the Kimball methodology.

Helper tables as far as I know have been used in many (often incorrect) contexts, one of which are bridge tables by people who fail to use the correct terminology.

Some people use helper table as a synonym for bridge tables, but helper tables also seem to have gotten misused in the context of a staging database or datawarehouses to name any table that helps something.

However, in dimensional modeling "helper table" isn't really defined, if it's used it's mostly as a synonym for "bridge table" (which is an actual term) or could be used to name about any table which "helps" achieving a goal but then there isn't a real definition.