Redmi is a sub-brand of Xiaomi. Their smartphones are in the entry-level with the Note series aiming at the mid-range sector. The K series—which came to live in 2019—offers flagship specs and is mostly focused on the asian market. Some of those K series models get re-named under the Xiaomi brand for the global market (e.g. Redmi K20 as the Xiaomi Mi 9T).
Official Xiaomi website with Redmi phones: https://www.mi.com/global/