(From highest end to lowest end.)

MIX is their flagship/concept line, where they innovate.

Mi is their main flagship line.

Black Shark is a gaming phone startup that Xiaomi provides funding, support, and production for. It is not a Xiaomi line.

Mi Note is their premium mid-range line, featuring specs comparable to the last gen Mi series devices.

POCO is a cheaper sub-brand that’s closer to mid-range but keeps the high-end SoCs.

Mi A is their mid-range Android One line.

Mi Max is their mid-range phablet line, specs comparable to Redmi Note.

Redmi is another cheaper sub-brand that focuses on mid-range and low-end devices, however, now that they’re a sub-brand, they plan on releasing higher end devices similar to the POCO soon. (Redmi Note is higher end than standard Redmi)

Edits
: added Mi Max line, added Mi Note line, clarified Redmi Note, clarified Black Shark