If you use an ESP8266 board, like a NodeMCU or Wemos D1, of course you can - those come with a built-in USB interface.

For many other devices, there's tuya-convert - creates a special hotspot which "captures" the device, then flashes it via OTA for you. But Tuya already changed the firmware once to block this, and although the newest tuya-convert works again, there's no telling they won't do it again.

Lately though Tuya devices have started using a Realtek RTL8710BN instead of the ESP8266 - those are completely incompatible boards, and to my knowledge there's nothing like Tasmota or ESPhome for them.

On Sonoff devices: https://tasmota.github.io/docs/Sonoff-DIY/#compatible-device...

Also notable is that you can get Tasmota-capable Wifi power sockets for about 10€ if you are willing and able to use some electronics tools. I have three of them with Tasmota. The first took about two hours to flash with some trial and error, the other two about an hour for both.

Depends on the devices. Some are possible to flash over the air. Have a look here: https://github.com/kueblc/tuya-convert/tree/new-api

I just flashed Tasmota on to three Sonoff S26’s without any soldering. I did need a USB to serial adapter to connect to the serial and power pins - I got one that included an ESP-01 so I knew it should work [0]. I also needed some bits of wire and crocodile clips, but that's all. The S20 is easier by all accounts, but I didn't know that until I'd already got the S26's. Having three to flash, I was motivated to figure out a solution that didn't involve soldering.

[0] https://www.amazon.co.uk/gp/aw/d/B07KF119YB

The ease or difficulty of re-flashing is dependent on the hardware. Some (most?) still need soldering, or at least a steady hand to hold wires in place, but the soldering on some things is far easier than others.

I've managed to flash half a dozen Sonoff light switches, as many power outlet switches, a whole bunch of Wemos D1 Minis, and even my kettle. Its generally straight forward and the first one is always the trickiest.

Initial OTA flashing might be a thing on some devices, but hasn't been on the Sonoff thing I have for a while. Once flashed though, you never need to touch the hardware again.

It's been a fun project or two.

For some devices, yes.

Tuyas are quite popular:

https://tasmota.github.io/docs/Tuya-Convert

Sonoff too, apparently depending on which firmware version they come with:

https://github.com/mirko/SonOTA