crucial jq insight which unlocked the tool for me: it's jsonl, not json.

it's a pipeline operating on a stream of independent json terms. The filter is reapplied to every element from the stream. Streams != lists; the latter are just a data type. `.` always points at the current element of the stream. Functions like `select` operate on separate items of the stream, while `map` operates on individual elements of a list. If you want a `map` over all elements of the stream: that's just what jq is, naturally :)

stream of a single element which is a list:

    echo '[1,2,3,4]' | jq .
    # [1,2,3,4]

unpack the list into a stream of separate elements:

    echo '[1,2,3,4]' | jq '.[]'
    # 1
    # 2
    # 3
    # 4
    echo '[1,2,3,4]' | jq '.[] | .' # same: piping into `.` is a NOP:

only keep elements 2 and 4 from the stream, not from the array--there is no array left after .[] :

    echo '[1,2,3,4]' | jq '.[] | select(. % 2 == 0)'
    # 2
    # 4

keep the array:

    echo '[1,2,3,4]' | jq 'map(. * 2)'
    # [2,4,6,8]

map over individual elements of a stream instead:

    echo '[1,2,3,4]' | jq '.[] | . * 2'
    # 2
    # 4
    # 6
    # 8
    printf '1\n2\n3\n4\n' | jq '. * 2' # same

This is how you can do things like

    printf '{"a":{"b":1}}\n{"a":{"b":2}}\n{"a":{"b":3}}\n' | jq 'select(.a.b % 2 == 0) | .a'
    # {"b": 2}

select creates a nested "scope" for the current element in its parens, but restores the outer scope when it exits.

Hope this helps someone else!