playerunknowns-battlegrounds
In Player Unknown battlegrounds there's a massive underground bunker in F4 filled with high grade loot. I know from a game where I joined a random squad that there are more underground bunkers as we ran across a small one as we were traversing the map.
I have not been able to find the smaller bunker again; how many underground bunkers are there and where are they located?
I did some tests and here are the results:
UAZ/Jeeps: 90+ with boost on road, 85-ish most of the time.
Note: The black cloth covering the UAZ does not block bullets.
Dacia: 120+ with boost on road, ~100 most of the time.
Buggy: 90+ on road, 85-ish mostly. Boost does not increase top speed.
Motorcycle(2-Wheels): 150+ top speed on road, easily hits 110 even on rough terrain.
Motorcycle(3-Wheels): 130+ top speed on road
Boats: 90+ with boost
There may just be coincidences with a lot of people having similar gear. I often see the same stuff because people aim to get the better gear and obviously level 3 gear will be the most benefit to you. The places with higher loot spawns, and better gear tend to have a high percentage of players dropping at so it's not unlikely you'll see a lot of players with that better gear.
As far as I known in my time playing, and from reading forums and threads about loot spawning, there doesn't seem to be any particular weight applied to certain things spawning in different matches.
Common loadouts are ARs with a shotgun or SMG, in my experience, which is why they're so commonly seen. The more powerful guns have less frequency so that also contributes to seeing more SMGs around the place.
Some early threads like these all mention how it's weighted towards certain zones having better loot, and the farther away you are from them, the more RNG is factoring in. In my experience the more isolated places tend to have some things in them, and a variety of gear, but there is the chance nothing/little amounts of things are there.
If you check out various loot spawn guides like this or this, they'll tell you that you're guaranteed better things at places like the Military Base, the School, or the Power Plant to name a few.
Here is an image which shows the general distribution, and as you can see, some places are much more weighted in the amount/quality of loot spawned. 
Best Answer
There is a set of very usefull maps made by ShatterNL.
You can see all the locations including the bunkers.
check out this one:
But be sure to check for map updates every month as the developers do sometimes change some things.