There are "just field recordings" and "pro field recordings", no matter what you record - city, nature, or anything else. Except the equipment you use (which is important) and some mastering you do later (which can be also important, but does not have to be) - what makes the real difference between "ordinary" and "pro"?
Let say you want to record some nature. Take out of your pocket your field recorder, and point it in the direction of incoming sounds. That's the "ordinary" recording. There are only a few percents of chace, that you did get nice catch.
The first important thing is - to find right place.
When you stand on the threshold of a forrest and hear some wonderful birds, blended with other things like insects, rustling leaves, wind, perhaps a brook - often you might believe, that when you come closer to that mixture of sounds, you will get better results. Nothing more wrong. You should begin to perceive the space around you in terms of... "sound boubbles". There are crossing points, where you get the best and unique results, but if you leave these areas - the beauty disappears. When you stand in right place, you hear certain sounds "from the distance", but only there where you stand - these "distand sounds" - sound the best.
I give you another example. When you buy so called "near field monitor speakers", like pair of KRK RP5 G2, and place them correctly - they create with your head an equilateral triangle (left channel, right channel and you - in the center and in front of them). Monitor speakers are specific kind of speakers, designed for proper sound mixing and mastering; such speakers have flat frequency response, deep dynamics and great detail level, but they also project virtually - a localization of sound sources, like instruments. Now here is a trick. Only when you sit in the "sweet spot" (your corner) - you can perceive projected sounds correctly in space - closer and farther, more left or more right, lower and higher, louder or quieter, smaller or bigger. You may get the impression, that "this talking head is 50cm behind the speakers, and if you get closer there, then you will be inside this singing head". But when yoy leave the sweet spot, and move your behind the speakers - you get nothing, just a regular stereo sound with no spatial properties.
It's as if the environment was producing a "holographic" and coherent experience only in certain, very limited area, and everywhere else - all sounds were just a chaotic sonic components (although they are still in space) of local trafic.
So when you find the a place, where all sounds blend together to one, well balanced space - this is your recording place; it will "just sound right". The waterfall - is as it is - only at this distance, only in "this there and now". Although many sound sources are omnidirectional, they have "directional shape components" which are projected in a certain way.
The second important thing is to find right angle/direction.
Many believe, that you get the best results if you get very close, and put your recorder into the direction of incoming sound. Take the "first important thing", and add some details. You don't seek for loudness, but for clarity, details, balance. When you go to do some field recordings - you should have any headphones to adjust your recorder to what you hear directly. You will find, that when you turn around, checking all of your 360 degrees (yes, your "behind" too), or make different up/down angle - eventually you find a position in which the recording sounds perfect. It does not matter what confing of mics you use (90 degree, 120 degree, ortf, binaural or else). It's like "being in the flow"; either you are with or against.
For example, when you record a small river waterfall or brook - it might sound better if you point your recorder upward. Finding left/right sonic compensation (other sounds) will help if this event is not directly in front of you or if it is asymmetric (in terms of sound pressure / intensity). Sometimes turing the recorder so that the 2 mics are up-down directed - helps to improve the stereo field. Another example. If you direct your recorder downwards - some sounds dissapear from your recording, but the whole recording can be damped a little bit. ou need to do some experimentation and be open-minded for available options.
Third important thing is about "boubbles".
Even in more dense places, where there is a lot of other trafic (like civilization or frustrated dogs), sometomes there are points, where unwanted sounds are filtered. How? As I said before - sounds are projected directional in some way. Objects like hills, trees, buildings or terrain formation (elements which interfere with each other in straight line between you and unwanted sonds) do the rest. Sometimes adjusting the angle of recording direction - can damp a little bit such sonic intruders. Sometimes it is a matter of few or a dozen meters, to get the big difference.
Knowing, recognizing and understanding first two important things - you will be able to get really pro nature recordings. Third issue will help you to find better places near the cities or other human activities.