Should the arpeggio be placed before of after the flat symbol?

Always before.

Source according to this book page 12 (pdf page 8):

htps://alpinevistamusic.weebly.com/uploads/2/3/0/7/23077310/essential_dictionary__of_music_notation.pdf

just draw the accidental all squiggly.

I have no info guess about this, but i strongly guess that the arpeggio sign goes to the left of a flat sign

From what I understand you always want to keep accidentals as close to their note as you can to decrease chances to misread the notation.

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