"How to Play," pg 3 of the basic pdf. Spells out the role of the DM in resolving, well, everything.
The worst thing about 3e (which 4e didn't solve, just kinda swept under the rug), was the attitude that developed in the community that valued RAW over DM judgement. 5e nips that in the bud.
Call it 'DM Empowerment,' it's the single best thing about 5e.
(Not that DMs couldn't exercise the same sort of power in any system, even any hypothetical system that came right out and said they couldn't - just that 5e comes out and says it from the very start, establishing a foundation for community acceptance of DM rulings and house rules*.)
(*ridiculous grognard digression: I don't know why we have to call 'em 'house rules.' Back in the day, they were 'variants.' And, published adventures were called 'modules.' Now published variants are 'modules,' so the game can pretend to be 'modular.'
bah! humbug... kids these days ... get off my law... :shakes cane: ... insert ageist stereotype of choice... etc..)