To elaborate on what 25or624 said, "nedenfor" can sometimes be used to describe something that is directly below you (on the floor below the one you're currently at, for instance).
However, it's also used to express that something is down past something else. If I say that my house is "nedenfor butikken", that means that it's past the store, but there's a drop in elevation from the store to my house. It's a more specific variant of "bortenfor", and its counterpart is "ovenfor" (not "overfor").
Not all letters in all languages are pronounced audibly. See for example English, which is notorious for this. I do hear a whiff of "r", hear, though it is very weak.
I am not sure, but without the "r", it might be that the "o" (pronounced more as "u" to my ear") would be pronounced as a long vowel.
This link has been in these discussions for a long time. Extremely useful: https://www.skapago.eu/nils/pronunciation-retroflex/
But "der borte"... it doesn't mean past the landmark, it just means toward the landmark, correct?
Bak/behind = at the back of something, e.g. Hagen ligger bak huset/the garden is behind the house. This is always true, as the position of the garden is relative to the house, and it doesn't matter where the speaker is standing.
Bortenfor/past/beyond = the other side of something, e.g. Huset ligger bortenfor skogen/the house is beyond the forest. This is not always true, as the position of the house is relative to the position of the speaker and the forest. If the speaker is in a different location, then it could be that "skogen liggen bortenfor huset/the forest is beyond the house".
Hun bor under butikken = she lives under the shop, i.e her apartment is in the same building.
Hun bor nedenfor butikken = she lives below/down past the shop, i.e. her apartment is in a different building from the shop, and that building is at a lower elevation/downhill from said shop.