how can I prevent apps from immediately connect upon wifi connection I often have to connect to public wifi networks, and upon connecting (automatically or otherwise), several apps will immediately connect to their host server and start exchanging data. How can I make sure that those apps will not start anything until I have my VPN turned on ? Even if I have my VPN auto-connecting, there is still a [sometime longer] delay during which other apps will start "chatting" on the net..........

I'd like to be able to give the go before apps automatically connect to the net. Some will let me configure a "connect on demand" but most will connect whenever they can.
Since Android 8 there's built-in option Block connections without VPN in VPN settings.

In other case you may use AFWall+ (root required) from F-Droid or Google Play to allow apps to connect to internet only through VPN (only paid version supports VPN).