I can’t open your AUP file because we don’t have the associated sound/data files. Can you give us a WAV or MP3?
Sorry if this is obvious, but the AT2020 is a side-address directional microphone. Make sure you are recording from the front side, not the end or back-side. (You didn’t say which CAD mic you were using. It may be similar.)
Different mics have different sensitivity. As Koz says, there is no volume/gain control on this mic. There is a trade-off between gain and maximum signal level. You can probably stick the in front of a guitar amplifier and hit about 0dB. So, I wouldn’t be surprised if you are getting -20db or less with an acoustic guitar. (0 dBFS is the maximum digital level from the ADC, and it might correspond to an acoustic loudness of 100 dBSPL or more depending on the mic sensitivity and preamp gain, etc.)
What CAD mic are you using? Maybe it’s a better choice for this application.
I am recording acoustic guitar w/o pickup and my mic is around 30cm away.
I am very new to recording, is there any settings I missed out, or maybe my mic placement is not right?
The distance seems about right. Probably the most common setup is to aim the mic at the soundhole-end of the fretboard. You’ll have to experiment for the best results, but you don’t normally aim the mic directly at the soundhole.
Using ‘effect’ → ‘amplify’ will get me louder volume but at the same time I hear buzzing sound.
The noise is on the analog side. There will be acoustic room noise and preamplifier noise (from the preamp inside the mic). You may be able to lower the acoustic noise, but there is nothing you can do about the preamp noise.
If you can increase the signal, you’ll get a better signal-to-noise ratio. The only way to do that is by playing louder, or by the mic closer to the guitar. But, both of those things will affect the character of the sound and may not be desirable.
The fact is, solo acoustic instruments (and solo vocals) are darn hard to record!!! The more “sound” you’ve got, the more you mask (drown-out) the noise. Professionals still record in soundproof studios with good low-noise equipment…
And if I compare the wave shape, using AT2020 gives me more of those steep slope. The volume distribution when fingerpicking and strumming is so different even though I played normally.
My CAD mic is also a USB type, plug and play. I recorded with the same method and distance. Sound quality wise, AT2020 is better, but it has low volume and sharp spikes on the wave, so it is hard for me to ‘amplify’.
Microphones are linear (until something clips/overloads/distorts). i.e. If you increase the quiet sounds by 10dB, you also increase the loud sounds by 10dB.
If the AT has better-stronger high-frequency response, the transient peaks may be higher, but otherwise the peak-to-average ratio should be about the same with either mic (unless the CAD is overloading/clipping). You can reduce the high frequencies with EQ, but then the recording will be more “dull” or “mellow”. You don’t want to do that unless it improves the overall sound.
You can change the frequency response of the CAD recording with the Equalization effect. It’s nice to have the perfect mic for every situation, but it’s easier & more economical to alter the “character” of the sound with EQ. I’m not saying you should use cheap microhones… Maybe the CAD isnt’ going to cut it… But, you don’t have to go overboard for home recording either.
Any methods I can do to increase the volume?
As you’ve found, increasing the volume will also increase the background noise (keeping the same signal-to-noise ratio). You can try some noise reduction, but if the noise is bad, the artifacts (side-effects) of noise reduction sometimes makes things worse.
Commercial recordings use LOTS of dyamic compression & limiting to booost the overall/average level without boosting/distorting the peaks. There is a Compressor effect, and an optional Hard Limiter effect. But, it will take some practice and skill and probably some more advanced effects/plug-ins to get the loudness of a commercial recording. And over-doing it will kill the dynamic expression, making everything constantly-loud and boring (like lots of current releases! 
).
And, using compression to boost the loudness will of course boost the background noise.
