Xiaomi has released MIUI 12 to the Redmi Note 9S, Redmi Note 9 Pro and Mi Mix 3 5G. The latter had already received the OS upgrade on the European branch of MIUI, but the new update is the first for global handsets. The update may bring MIUI 12 to more owners of the Mi Mix 3 5G, but Xiaomi continues to palm the device off with Android 9.0 Pie. In fact, the Mi Mix 3 5G is one of the few smartphones to receive Android 9.0 Pie builds of MIUI 12.
It is worth bearing in mind that the update, called V12.0.1.0.PEMMIXM, is currently a Beta Stable build. This means that Xiaomi is pushing V12.0.1.0.PEMMIXM to select devices to rule out any last bugs before it issues it to all handsets. However, you should be able to install it manually via the link below through recovery mode.
Similarly, a Beta Stable build of MIUI 12 is available for the Redmi Note 9S and Redmi Note 9 Pro. V12.0.1.0.QJWINXM can be downloaded manually but only for devices that been enrolled on the Indian branch of MIUI. V12.0.1.0.QJWINXM is based on Android 10 for reference, unlike V12.0.1.0.PEMMIXM for the Mi Mix 3 5G. The inaugural MIUI 12 update for the Redmi Note 9S and Redmi Note 9 Pro should denote the beginning of MIUI 12 upgrades for other branches of MIUI. These should include the global, European, Russian and Turkish branches, although it has been months since Xiaomi upgraded the Redmi Note 9S in the latter region.