When a potato cooks, the starches contained in each cell are released as the cell walls break down. These starches absorb the potato’s internal moisture and swell and soften. These two processes are what transform a raw, hard potato into a cooked, softer potato fit for mashing.
If you cut your potatoes up before boiling them, the starches absorb the internal potato moisture as well as the water in which they’re being boiled. If they boil for too long, they absorb too much water and your mashed potatoes will become gummy. The difference between perfectly cooked potato pieces and soggy pieces can sometimes be as little as a minute or two.
An easy workaround is to boil potatoes whole. They’ll take a bit longer to cook but you can leave them in the hot water after boiling without undesirable effects, keeping them warm until it’s time to mash them. I start my potatoes boiling as soon as I begin cooking and mash them immediately before dinner.
If they need more moisture, you can add a bit of hot potato boiling water or another liquid. This way, you have more control of their moisture content.
If you don’t know how to eyeball milk right, sure. If you know what you’re doing you’ll actually introduce a lot more air and make them fluffy. I’m kind of flabbergasted by how these folks are screwing up mashed potatoes, because you are literally standing there and adding ingredients until they feel right to you.
I usually use a masher, but a mixer will produce objectively better texture.
I wouldn’t saw it is objectively better with a mixer, but both chunky mashed and properly whipped potatoes are awesome for different reasons.
This is totally wrong? It has nothing to do with milk, you’re just overworking the starch. Use a ricer to do 99% of the mashing if you need extremely smooth mashed potatoes, then you can try to whip in a tiny bit of air at the end if you want, but just butter + milk + folding is enough