Regex with backslashes in a collection

I'm using Vim 8 or 9 with 'magic' and re=0 (same using re=1).

I want to find "\[" or "\]" in text like:
abc\[def\]ghi\\xy\z

The following pattern also find the two backslashes.
\\[\[\]]

Help says that a backslash in a collection (if not followed
by certain characters) just means a backslash.
:help /\]

Why does the pattern find two backslashes (but not one)?

These patterns find only "\]" (good):
\\[\]]
\\[]]

But this finds "\[" and "\\" (not "\"):
\\[\[]

John

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