It’s been going on for a while. Search YouTube or the web for 48gb 4090 (this is one of the most popular modded Nvidia cards), Nvidia of course never officially made a 4090 with this much memory.

There are some on sale via eBay right now. The memory controllers on some Nvidia gpus support well beyond the 16-24gb they shipped with as standard, and enterprising folks in China desolder the original memory chips and fit higher capacity ones.

I've seen a guy who sells modded 2080 Ti with 22gb for $500

https://www.tomshardware.com/pc-components/gpus/chinese-work...

There's also unreleased Nvidia engineering samples of cards with doubled VRAM like this - https://www.reddit.com/r/nvidia/comments/1rczghu/update_unre...

Go at ebay and search for RTX 4090 48GBs. There's plenty of them with prices around $3.5k