Q:

How Roman Picisan Constructed

This is one of the "sacred" song by one of the legendary band in Indonesia. The song is called Roman Picisan. I want to know more how this song constructed especially in the verse section (what mode or scale this song in). I have limited knowledge in music theory but by using my ear here's what I got
Verse (0:52 - 1:20)
|Am . . .|E . . .|Am . . .|E . . .|
|Dsus2 . Bb/D .|C . G .|Dsus2 . Bb/D .|C . G .|
|Gmaj7/D . . .|Dm . Am .|E . Am .|
Reff (1:20 - 1:37)
|A . . .|C#m . . .|Em . . .|Dm . . .|
|A . . .|C#m . . .|Em . . .|Dm . . .|
I got two questions, first is in what scale or maybe the mode is the verse in because it's a weird chord progression for me. Second, how the modulation works from verse to reff because the verse is ended with Am chord but it's suddenly jump to A chord and it's still sounds good (for my ear).

Link for the song: Roman Picisan (Song starts at 0:50)

A:

A lot of things are going on in the song!
The verse is in A minor, it starts with a simple Am/E vamp.
Then it plays Dm/ Bb/D / C / G. The only "strange" chord is Bb/D, Bb in first inversion that is. It is the bII chord of the A minor scale and is called the neapolitan chord.
It creates a very beautiful sound, because it encloses the A note between the Bb (one half step after it) and the G# of the previous E chord (one half step below A). This chord does not strictly belong to the A minor scale because Bb is not diatonic to it, but we just explained why "it works".
C and G are just diatonic to the key, nothing strange here.

In the chorus we modulate to A major. This is common in a lot of songs and pieces (Why my guitar gently weeps by The Beatles, first that comes to my mind, or Time in a bottle by Jim Croce). It gives an uplifting feeling to the song, because it changes the mood by staying in the same range at the same time.
The modulation works by just doing it ^^. Some pieces in minor actually end on the corresponding major chord to give a relieving feeling - this device is called tierce de picardie, but does not apply in this case because it is not the final chord of a section, but the first of a new one.
Actually the mode is not strictly A major, but A mixolydian because of the Em chord - the fifth chord of the mixolydian mode is minor, as opposed to the major/ionian where it is a dominant one (V7).
This twists things a bit by making them less obvious. Then we have a iv chord, D minor, that should be major in both ionian an mixolydian. You can see it as a borrowed chord from the parallel minor we were in before (A minor), so we can transition better to the previous key.

