JavaScript (ES2022), 141 bytes

s=>a=>"    "+s+s.replace(/(?:[^+-.<>[\]]+|.)(?=(.*))/g,(c,e)=>``+(c+e.replace(/.|$/g,'')).padStart(s.length+5)+a.at('><+-.,[]'.indexOf(c)))

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f: (source_code: string) => (explain_array: string[]) => string