Every month, Steam surveys PCs at random and publishes its findings publicly. The company polls for things like clock speed, monitor resolution, and GPU vendor, brand, and model. Though Steam's survey isn't perfect and may not be representative of the market as a whole, it has generally been accepted as a good representation of what gamers are buying.
The video above shows GPU share based on Steam's historical rankings. Steam's most recent survey has seen the RTX series gain market share faster than AMD's RX 500 and Vega series (which includes the 56 and 64 but probably not the VII), as well at the expense of Nvidia's previous generation 10-series.
Of Nvidia's RTX 20 series, the 2060 gained the most share in March, followed by 2070, then 2080, and finally the 2080Ti. All of the RTX models have individually surpassed the RX Vega series, even the 2080Ti, which finally solidified its lead against Vega in March after tying in February. Despite Vega being much cheaper and offering better value, RTX seems to entice gamers more.
However, AMD has also made gains thanks to lowering prices on the RX 500 series. Since November, the RX 580 has been AMD's fastest growing model and one of the fastest growing models on the entire survey. Considering that Radeon GPUs are rarely in laptops and that a large segment of Steam users' game on laptops (which is why Nvidia's mid-range 10-series GPUs lead the survey), it's rather impressive that AMD has nearly doubled the 580's market share in such a short amount of time. However, the 580 is still behind GPUs like the 1070Ti, which has sold more than the 580 overall despite being far more expensive and less mainstream. The 580 has also not managed to outsell the 970, which is now two generations old.
On the other hand, the 10-series, of which the 1070Ti is a member, has been shrinking in size. Although the GTX 1660 and 1660Ti are not on this survey, it is likely Nvidia's previous mid-range 10-series GPUs are losing market share to these new GPUs, as well as the slightly more expensive 2060. Overall market share has not changed significantly between Nvidia and AMD, standing at 74.75% and 14.9% respectively. Though RTX is a fast-growing segment for Nvidia, it is likely that users buying RTX previously owned 10-series hardware, offsetting the gains. Meanwhile, AMD is gaining share from gamers looking to upgrade on the cheap.