The song has five distinct parts
"I've been cheated by you since I don't know when"
"Look at me now, will I ever learn?"
"Just one look and I can hear a bell ring"
"Mamma mia, here I go again"
"Yes, I've been broken-hearted"
This followed by a reprise of the "Mamma Mia" section to conclude the multipart section.
You may even argue there is 6 sections if you included the instrumental intro

Now what makes it exceptional isn't that it's 6 distinct section join together or that each section is individually memorable. After all The Beatles medley has done that before

It is exceptionally is how smooth it goes from one section to the other and it doesn't sound like a multi-part song and it doesn't sound like a medley whatsoever. It sounds like a singular song with 5 parts rather than 5 part smashed together.

The closest The Beatles achieving this is probably You Never Give Me My Money but that still has a medley vibe to that.

With the song Mamma Mia, ABBA managed to outwrite The Beatles (or any prog multi-part epic) which is an enormous achievement.

One of the all time greatest pop song of all time IMO.

What I also like is how compact it is and that the 5 sections (plus the intro) is put together in 1 minute and 20 seconds.

I kinda wished there is a version of a song where ABBA removed the single repetition in each of the part and would have bring compact songwriting to further extreme and put in those 5 sections in 40 seconds.