I have total aphantasia and this is how I usually answer: it's like having a picture of my mother in my head, only for it to be obfuscated by an opaque veil, and there's a system in my mind that can peak behind it and feed me informations. In the end: I can never visualise her, but I can "query" informations, and when I see her I match her immediately with what I know her to look like.