What are the supported radios for now?

[lattice0]

Which radio and firmware I should buy/flash to test this project on?

[Nerivec]

See https://github.com/Nerivec/zigbee-on-host?tab=readme-ov-file#testing
Nordic is still TBD.
Others are less common, so, we likely won't know until someone decides to dig into them specifically.

[lattice0]

so open thread is a firmware that also supports "sniffer mode"?

[Nerivec]

OpenThread RCP has several capabilities to manage the radio. Here we use the ability to send raw MAC 802.15.4 messages with STREAM_RAW property of Spinel protocol.
This post may interest you: Koenkk/zigbee-herdsman#1308 (comment)