Will try to gloss over many details to keep terse.

That's for 2G. 3G and 2G are currently being phased out in favor of 4G and newer. Even many trail cameras will soon no longer work.

That antenna setup will only help if a strong signal is available immediately outside but blocked by your dwelling. Which is unlikely unless you have a steel roof or cinderblock walls.

Adding an antenna is going to attenuate the signal. Going through a connector is going to attenuate the signal. Going through another antenna is going to attenuate the signal. The result is a signal significantly reduced in db compared to outside. That means the signal is moved closer to the noise floor.

3G and newer are highly time sensitive protocols. They have GPS clocks to synchronize signaling across towers. That setup could potentially place signals out of phase and time synchronization. Meaning even with a signal the error rate and potentially constantly tower handoff negotiation may prevent effective communication. Additionally, if you currently get a signal indoors, you may create what's regarded as reflection and echo, effectively causing other RF issues. Including self cancellation with the magnitude of the outdoor signal, assuming they remain in [inverted] phase.

In a nutshell, there's a reason boosters are sold. Even if that DIY works (increased signal) it's going to be niche in where it actually works well. While I'm not saying don't, I wouldn't hold my breath.

It talks about 2G service, which is almost gone in most places. In the USA, only T-Mobile still has some and that's because of crazy people like me who still have 2G M2M devices on the air.

However, antennas don't really work like this, you're not going to receive a signal and re-broadcast it in any meaningful way without some sort of amplification. Antennas do have a factor called gain, but that's related to how efficient the antenna is and how effective it is at aiming the radiated power at the target.

It also says this:

Somehow you also can use your phone headphones work as an Antenna too, many people over look this.

No. The headphone cable is used as an FM antenna for built-in terrestrial broadcast radio, not for the cellular signal. This person is an idiot or a troll.

You can build an antenna/receiver like this that will boost signal if there is signal to be boosted.

There are several companies that build options and several options offered by companies.

If you have at&t call and ask about a microcell.

Verizon has one too but idk the name they use.

Williams wireless makes pre-made systems for this.

Or you can build your own.