Addressing the price difference between RTX 3090 and 6900 XT:
3090 is priced at $1500 for its 24GB RAM which enables ML & Rendering use cases (Nvidia is segmenting the market by RAM capacity).
AMD's 6900XT has the same 16GB of RAM as 6800XT, with less support on the ML side. Their target market is gamers enthusiasts who wanted the absolute fastest GPU.
3090 is priced at $1500 for its 24GB RAM which enables ML & Rendering use cases (Nvidia is segmenting the market by RAM capacity).
AMD's 6900XT has the same 16GB of RAM as 6800XT, with less support on the ML side. Their target market is gamers enthusiasts who wanted the absolute fastest GPU.