[PhantomGremlin]

The following is relevant to your comment, since you I think are talking about a standard, conventional setup. In contrast, the article has a weird setup for an apartment.

There are adapters you can use to work around the lack of wires. The existing wires already include power, it's the ground that's missing.

The existing 4 wires to a standard dumb thermostat are:

Power

heat

A/C

fan

The functions are enabled by connecting power to whichever is required. But there is a missing "C" (common) wire. So, as you discovered, smart thermostats have no easy way to also power themselves.

I was lucky when I added my smart thermostat. It turned out that my 20 y/o house already had an 8 wire cable preinstalled.

For people stuck with 4 wires, there are adapters available that will multiplex two control signals onto one existing wire. This lets you free up another of the existing wires to become the needed fifth C wire.

Here's one setup I found with a quick search. There's even a video. This adapter requires access to both ends, viz. the thermostat and the furnace.

https://smartthermostatguide.com/c-wire-venstar-add-a-wire-a...