I have seen a number of people claim that the game ‘assumes’ a certain score in a certain stat.
the default assumption is that scores are rolled.
just curious. I assume people put a good score in main/attack stat, but where has that been explicitly stated?
additionally, I have seen assertions about the math of the game likewise assuming certain scores in certain places.
any specifics would be great. Common sense says bonuses are good but where is that written? Just curious as the game seems to be less lethal than some past editions...
Hiya!
Bad assumption.

Different groups have different focuses. In my original, longest standing group (of which I still play with some of them; this is from the mid-80's; two players are from the beginning in '81, but haven't played with them in over 2 decades)...anyway....where was I? Oh yeah, my original group tended towards just picking a race/class that "grabbed their attention" after rolling their stats (3d6, in order; then 4d6, in order; then 4d6, wherever; choice is up to the player to choose which method to use). I've seen Fighters with 4 Strength (Basic D&D), Wizards with 13 Int but 18 Str and 17 Con (Basic D&D), Thief with 12 Dexterity (1e), and Clerics with 11 Wisdom and 6 Charisma (1e). And many, many in between.
For 5e, I honestly don't think Stats make a HUGE difference. Oh, for the record, we don't use Multiclassing, Feats or any of the Optional books (except on a case-by-case basis); meaning that a Fighter with a 12 Str and one with an 18 doesn't make that much of a difference...the ability to "min/max" is mitigated more than enough.
The core game assumes NO Feats, Multiclassing, etc. Most 5e DM's I talk to and read about make a HUGE mistake in allowing "everything under the sun" in terms of "optional books and whatnot" (Sage Advice, additional races/classes from adventures, Feats, Multiclassing, etc)...and then don't do anything to 'fix' all the things that just broke; typically monster/NPC opponents and skill DC's. So you get "this is OP" or "that is OP"...yeah...because the DM didn't add +100 to the other side of the equation, so now the PC's are all at Base Line +100 and the monsters are still at Base Line.
^_^
Paul L. Ming