If you buy the watch and have Android >= 8.0, you will probably have to update the software on the watch first, otherwise the connection will not work. I first connected with a mobile phone with old Android, carried out all updates on the watch, then did a factory reset and connected with the new mobile phone.
You can also do this without the old phone using software that you can get from the manufacturer. I read this online, but I haven't tried it out.