GPS only Apple Watch depends on the iPhone radios. So if your iPhone is turned off or is in Airplane mode, then If AND ONLY IF, your Apple Watch is on WiFi, which generally means you are at home, the Apple Watch can do most things via WiFi, if the iPhone is not available.
Phone calls with the iPhone out of action, ONLY if your iPhone’s cellular provider supports WiFi calling, and you have enabled WiFi calling on your iPhone, and you enable WiFi calling on your Apple Watch.
Internet based messaging, such as Apple’s iMessage will work if the iPhone is out of action, 3rd party internet messaging IF AND ONLY IF, the 3rd party service supplies an Apple Watch app (not many do, so if you depend on WhatsApp, you are out of luck).
SMS TEXT messaging depends on the iPhone’s more capable cellular radio, so no TEXTing with Android users if your iPhone is not available if the iPhone is powered off or in Airplane mode.
Cellular Apple Watch is similar to the GPS only if the iPhone is turned off or in Airplane mode, with the exception that it can make/receive phone calls away from WiFi, and any apps that get data from the internet, such as weather, etc… can use the Apple Watch cellular radio.
But SMS TEXTing and 3rd party internet based messaging apps have the same restrictions. SMS TEXTing requires the iPhone’s better cellular radio to be accessible via the internet, so it has to be powered on, logged into iMessage, and able to talk to the internet, but it does not need to be with you. 3rd party internet based messaging can use the cellular radio, but the messaging service must provide an Apple Watch app (which WhatsApp does not do).
The Apple Watch is an iPhone accessory, not a replacement.