As you see, lots of contradictory advice making a basic understanding difficult to grasp. The solution is to educate yourself by reading the science and then proceed with some confidence. It is a science but you can start by addressing the most important treatment which will get you a long way in the right direction.
I have posted a fair bit on this so you could read some older posts of mine. Once you have heard the amazing improvement you can then decide if you want to take it further by measurement which is inexpensive. For the cost of a beer and hamburger you can get a microphone and then download REW for free.
Know also that every room needs treatment. It does not depend on your type of furnishings etc. Ignore those that say: "I just use a rug and drapes and my sound is fantastic." All that tells you is that they have never heard a properly treated room.
Whether you use absorption and/or diffusion depends on the size of your room as does the required time for the sound to decay. This decay should be as near as possible across the spectrum and is known as RT60, being the time it takes for the sound to decay by 60dB usually about 300ms. All this info is on the net.
If you approach a professional acoustician and they don't ask for room dimensions just run away.
Small rooms require mostly absorption
Mid sized rooms require both
Large rooms require mostly diffusion
And all rooms require bass traps. In fact if you only do one thing then put bass traps in the corners. A floor to ceiling BT in the 2 front corners will have a huge effect. Google super chunk DIY BT's You will unavoidably have some nasty peaks and nulls. This is where the BTs help, they smooth out the room response. Get this right and you no longer hear the room, you hear the recording venue.
Next: Add broad-band absorbers at first reflection points.
The problem with a small room is the inability to place the diffusers far enough from your ears.