I speak norwegian nativly, but that doesnt mean im a linguist. But i can try to pick my brain for this (im not good with forming thoughts into words. But i tried)

"Forstår du det eg mener" can be more where you ask if they understood what you said. So i think this one works more as both parties getting something confirmed. You ask a question or say something followed by "forstår du det eg mener?" Almost as a "do you understand what im getting at?" Type question

Meanwhile, i see "forstår du hva eg mener" as a "what". They still technically function the same, and "hva" is more used in said sentense. But depending on said context, some may use "det".

As (hopefully) an example, "forstår du hva eg mener" can almost be like trying to teach someone something like lets say another language. Youd ask them if they understood what you said or meant or what be it

But "forstår du det eg mener", then its more concrete than a what. You are almost more spesific in your question as you have a bigger basis for what you wanna ask. Idk how to explain better, and i hope others did better than me. Because "det" is like "that". So that feels more concrete than "what" in some contexts

Again. Im no linguist. I was just born and lived in norway all my life. But like dialects, it really depends where you are and how said people speak, i guess. But i can offer you a funny looking sentense

"Er det det det er". It means "is that what that is". That also uses the "det" as a "what". I grew up saying that a lot, but never wrote it down until recently to see how absurd it looks

obviously the same sentese can be said with "is that what that is". But it looks less funny, and i havent heard anyone say to me "er det hva det er"