Q: What is a Companion/Slave?
A: This is a watch that allows you to read notifications on your watch receiveed by your smartphone. This allows you to read messages or view notifications without taking your phone out of your pocket.
Q: What is a stand-alone smartwatch?
A: A stand-alone smartwatch is a watch that allows you to use a SIM-card in it. This way you will have all the functionalities of a regular smartphone on your wrist like making and receiving calls, sending and receiving texts, planning events etc.
Q: What is the difference between stand-alone "Android OS" and "other OS"?
A: Watches with the Android OS are capable of pretty much everything you'll find in the Play Store. So this means you can install any app you want on it. Watches with "other OS" do usually not include a special app store, so they're limited to apps that are already pre-installed on the watch.
Q: I would like to buy a smartwatch as secondary phone, but I don't want to switch SIM cards all the time?
A: That's not neccessary, you can simply ask your provider for a "Duo SIM" card. This way you can put a SIM card in both phones. This way you don't have to switch SIM cards when you switch phone, but still keep the same phone number.