3 questions to start with Home Assistant

Hello everyone, i´m thinking on starting my instalation of HA in a Rpi3 i already have.

Can someone please answer me 3 things please?

1- Does Tuya sensors, wifi and zigbee, need to be flashed with some kind of firmrware? (like tasmota in sonoff products). If yes, is it reversible? And in wich website can i learn more about?

2- In case i have to change firmware, do i loose acess to sensors in Tuya app. I only can control them via HA?

3- For instalation of HA in a Rpi3 wich is the cheapest zigbee usb pen i can buy? I´ve seen some in ebay and aliexpress but i dont know wich ones to choose, what should i verify to buy a good one?

Thanks for the atention.