Start at က, work your way through Unicode in order: https://en.wiktionary.org/wiki/%E1%80%80#Translingual Some helpful Mon nationalist even made a writing animation for most letters before he got banned from Wikipedia. Fun fact: ပ is made with two strokes from opposite directions instead of a single one like you might naively expect.
I am fairly sure this is because, in many abugida scripts, the dominant writing material was palm leaf and the tool was a sharpened point or curved scalpel-like knife rather than a pen. If you curve inward from both sides, you don't have to contort your hand in a 180 degree pattern, which runs the risk of tearing the palm leaf. This is also why the shapes are all curves instead of hard 90 degree angles.
I've read that theory, too, but ဂ is written with just one stroke and if you look at palm-leaf manuscripts in general https://en.wikipedia.org/wiki/Palm-leaf_manuscript there are plenty of angled shapes. (Ironically, the example Wikipedia uses for Burmese has a more square style instead of the now-dominant curves.) So I wonder whether it's all just a post-hoc rationalization for historical happenstance. It would be interesting to know whether e.g. Sundanese ᮕ is similarly written with two strokes.
So many of these scripts basically derive from Brahmi which was famously rock-cut (Ashoka monuments). That type of medium is well suited to angles. Major scripts including Tamil were also enscribed on to metal .. this image shows an evolution directly away from angular toward rounded. https://www.instagram.com/p/CpB1ygEPwJt/
Presumably this reflects a shift from courtly scribe closed-door literacy toward more popular literacy specifically including temple-trained monks who presumably could not have afforded either stonemasons or metal plates for general use.