In the publ-imp-apa.mkvi (is it already VI?) I found the key line which I
would like to modify:

\startsetups btx:apa:list:book
    \texdefinition{btx:apa:authoryear}
    \texdefinition{btx:apa:title-if-not-placed}
...
\stopsetups

This, as I understand, determines the order in which the items are listed.
I need to: 1. place the year in the end 2. remove parentheses.

Do I start with btx:apa:authoryear and redefine it?

Do I use \texdefinition{btx:apa:suffixedyear} separately?

and:

in the following, where is the parenthesis?

\starttexdefinition unexpanded btx:apa:suffixedyear
    \btxdoifelse {year} {
        \btxflush{year}
        \btxflushsuffix
        \btxdoif {month} { % month and day optional in publ-imp-apa.lua
            \btxcomma
            \btxflush{month}
            \btxdoif {day} {
                \btxspace
                \btxflush{day}
            }
        }
    }