@Harmodius  Welcome to the community. And please refer to the following information below:

1. My question is, when I connect my phone and create a wireless network, what's stopping someone else to download the TP-link app and connect to my router as well - i.e. creating new networks?

When Deco is initially powered on, it will broadcast a default open SSID only for setup(since the smartphone needs to connect to this Wi-Fi in order to find Deco on the APP). After following the configuration guide on the APP, the previous open SSID will disappear and be placed by the new SSID you have created during the configuration. 

2. Will my "smart devices" connect automatically to my network if I haven't let them? You know, the fridge, washing machine, furnace, etc. I don't want them and don't expect them to, but since the Android connects automatically I am wondering if this is something I have to keep an eye out for?
No, the smart devices will not connect automatically to the new created network if you haven't let them.
3. Can I use the "satellite node" (to me this means the second M4 in the box I am getting) to connect a LAN cable to a desktop PC not a laptop.

Yes.

4. Can you recommend general tips about safety? I am already using strong passwords, VPNs and anti-virus software on my PC, but just anything I should know.

On the Deco APP, it is allowed to change the network security mode and set up your own password. After making sure all the devices are connected to the network, you could also hide your network from the available wireless network around.