Yes, all derailleurs pretty much look the same. To identify a derailleur, I need to look at the label on the derailleur. It should have a manufacturer name and model name. For example, see the label on the picture of the Shimano Deore derailleur at the link below:
https://bike.shimano.com/en-US/product/component/deore-m6000/RD-M6000-GS.html
The derailleur just gives me an idea of the level of components on a bike. If you see a bike with an XT derailleur on it, it’s probably a high-end bike. If you see a bike with Shimano Tourney, it’s probably near the bottom.
For example, consider 2 bikes, both Giant Talons (just for example, not necessarily recommending).
Giant Talon 3 -
rear derailleur - Acera
That tells me that the components on this bike are low-middle, which is a decent entry-level for serious mountain biking
retails for $700
Giant Talon 1 -
rear derailleur - Deore
That tells me this bike is higher-end than the Talon 3 - solid middle range bike. For someone a bit more serious about their riding - looking for lighter, more reliable components.
Probably the same frame, but if you look at the specifications, almost every component on the Talon 1 is upgraded over the Talon 3. Better fork, better wheels, better derailleurs, shifters, brakes, etc.
retails for $1200
Note that sometimes manufacturers specify a higher group rear derailleur than other components on the same bike. For example, they might have a Deore (higher groupset) rear derailleur, but use Alivio (lower groupset) shifters and other components.
For example, the Specialized Rockhopper below uses an SLX rear derailleur, but the front derailleur and shifters are Deore, one tier down.
In general though, a new bike with an SLX (higher groupset) derailleur will have better components (and cost more) than one with an Acera (lower groupset) derailleur.