We don't know that he is "low ranking" Finn is an odd trooper, or at least our conception of a trooper.
He says he was "in sanitation", however he was part of a ground assault on an important mission.
This could indicate that the First Order have a policy of having their entire work force combat ready, which would not be abnormal considering the rest of their depiction.
That alone indicates that their training and duties are probably not specialized but multi-faceted.
Well, multi-faceted in the strict confines of Starkiller given his lack of linguistics and basic ship mechanics.
More importantly, there are hints in the movie that he isn't just a basic grunt.
While he's clearly no officer, he: Again, is one of the few (comparatively given the size of the standing force on Starkiller, although apparently the his entire class considered above average) chosen for an important mission.
Is familiar with and is known by Captain Phasma.
Apologies for the second hand citation
(I'll shoot for an update later), but she mentions "FN-2187 has the potential to be one of the finest stormtroopers I have ever seen" in the novel.
Out of all of the people on the base, Phasma seems to have an eye on Finn (although @DVK notes it might be an eye of suspicion).
She probably doesn't ask for his blaster randomly but rather because she was actively keeping an eye on him during the raid.
In other words, Finn was an up and coming member of the First Order.
Since his first instinct to take down the Starkiller (or more accurately, to find Rey) is to go right to Phasma, it likely that his understanding of the base comes directly from her.
scifi.stackexchange.com/users/22987/joshbirk
Finn does not know many details about the Starkiller Base , He only knows the 'base'-ics.
The Starkiller base is the size of a planet, it would not be uncommon for anyone on it to know where major points of interest are, such as Central Command or a Thermal Oscillator , just like you know where Canada or Australia are on our planet.
Finn also mentions that he helped build it, and if he worked in sanitation he must have seen maps to get around to clean specific places.
He did not know how to disable the shields, he needed a higher up, Captain Phasma to disable the shields for him.
So he did not know very many details regarding the base such as codes.
In the attack plan, the Resistance are talking about how they need to blow up the Thermal Oscillator, all Finn does is point to it and arbitrarily say there will be enemy fighters.
I think Finn just happened to know the right amount of information to help the Resistance, but no great amount of details above his rank.
scifi.stackexchange.com/users/6322/hammythepig
I suspect that this is a case of lazy writing.
I mean . . .
the movie started out with the search for a map that could lead to Luke Skywalker and it became all about destroying the First Order's Starkiller base.
To make matters worse, why would a character who had been taken and raised to become a stormtrooper, conveniently had a job as a sanitation janitor.
Or perhaps he was lying about being a janitor.
scifi.stackexchange.com/users/18984/user18984
Finn lies to the resistance about his knowledge of the base.
When they arrive at the base, he explains he was in sanitation as the reason he doesn't know that much about the base.
From here, the team comes up with an ad-hoc way to deal with the machine.
Finn knows about large structures and their basic purpose and is able to say "Yeah, we got one of those."
when asked directly about it.
Is there more to this story?
Probably.
But we don't know any of it yet.
scifi.stackexchange.com/users/51/DampeS8N
Finn might be an example of how things are not always as they appear.
He may have been placed there by another authority to test ethical conduct or valour of its team.
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Most of it is from overhearing things.
There are quite a few cases where he says as much in the novelization.
A lot of the technical knowledge was gathered after the Resistance surveyed the planet.
He also was bluffing the resistance quite a bit in order to be able to land on the planet and save the Girl.
For example, he doesn't even know where the shield generator control room is until they find Phasma.
scifi.stackexchange.com/users/1132/Jeremy French
When the rebels are planning to attack the Starkiller base, Finn seems to know a lot about the security.  And yet, later on, he mentions he was in the sanitation department.  His rank was similar to a Private in the modern army. Would we expect a private to know all the details of how a nuclear base (for example) is guarded? So how come Finn knows all those details?
How does Finn know so many details about the Starkiller Base?
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<star-wars><the-force-awakens>
scifi.stackexchange.com/questions/114486
scifi.stackexchange.com/users/7931/Shantnu Tiwari
It is not clearly explained how Finn knows such detail about the Starkiller Base and it's quite possible that he was largely bluffing. However, nor is it clear what his original rank on the base was, and it's possible that he had much more information than we are led to believe. Finally, given that the Starkiller Base is the size of a planet, it wouldn't be difficult for anyone to know or overhear information about signficant points of interest. There are many possible explanations. For example Finn is not just a low ranking trooper, Finn does not know that much about the base, it is just lazy writing, Finn is lying about his knowledge, Finn is a kind of internal spy or Finn picked up his knowledge from overhearing things.
Finn is not the low ranking trooper he appears to be - there are plenty of signs of this. Finn does not actually know that much about the base - just where the main sections are. This is lazy writing - his backstory is too convenient to be believable. Finn lies about his knowledge of the base - and explains he was in sanitation to support his lies. Perhaps Finn was not a sanitation worker but a kind of spy installed to check on the behaviour of other team members. Finn picked up his knowledge from overhearing things - and there are still gaps in his knowledge.