多 + stative verb asks where a quality sits on a scale (多高 = how tall); 多少 asks a count or amount of a thing (多少錢 = how much money). One probes degree, one probes quantity.
多高 asks an absolute degree with no second party; 比 measures A against B (他比我高 = he is taller than me). The degree question wants a number; the comparison wants a winner.
splitting 多 from the verb: 你哥哥多很高 → 你哥哥多高 (the degree slot is the verb's own, no 很)
using a count word for a scale: 你哥哥多少高 → 你哥哥多高 (多少 is for quantities, not degrees)
treating 多高 as a fixed degree rather than an open one: it asks how tall and waits for a number, so it needs a question, not a bare statement
English fronts a how + adjective phrase (how tall is he); Mandarin keeps 多 glued to the stative verb in place (他多高), no fronting.