In fermentation the initial heat is usually to harm any microbes other than your preferred ones so the selected microbes have time to make the environment inhospitable to other microbes before they can do the same to the selected microbes.

Vinegar and alcohol in grape juice are two factions fighting for supremacy by trying to poison each other to death.

80c is a convenient temperature for activating naturally occuring enzimes in food for breaking down complex sugars. Similar, but food specific, holding temperatures are used for beer brewing, french fries, and other foods with complex sugars.

Been making yogurt at home for years. I just heat milk to the point when it's just about to boil over. Never measured the actual temperature, but I'm pretty sure it's less than 100C. I just cool it until it's warm to touch (about 40-45C) before adding the culture (a spoonful of last day's yogurt), then leave it overnight in the oven with the pilot light on.

I find that the yogurt is thicker when I initially heat it higher. My understanding is it changes some of the proteins, resulting in a higher curd yield.

It's pasteurisation, your 50C-only yoghurt would potentially not last as long; possibly even succumb to unwanted bacteria before (or the wanted be outcompeted by the unwanted) turning to yoghurt.

I don't measure it to 80C, just heat milk until it breaks then switch off and let it cool. I didn't have a thermometer when I first made it, but now I might measure that it had cooled enough. I wouldn't say it has to be as hot as 50C, you just probably don't want it hotter, so if you're making it without just let it cool to tepid, feeling warm to the touch, then add whatever you're using. Cheese similar, just more specific cultures.

(And I suppose if you really get into it, the specific temps and holding them thing is a lot more true for cheeses - or rather for different types - than it is for yoghurt.)

You should follow the guidance and use a candy thermometer to kill the nasties.

Once you make a few batches, you can usually eyeball it, different milks will act subtly different at temperature. Heating also changes how milk components can consumed by the cultures. I get milk from a farm that doesn’t homogenize it the same way as store stuff - the skin develops on the surface sooner.

Personally, I prefer to use a yogurt maker that keeps it at a consistent temperature. But you can make great yogurt in a variety of lower tech scenarios.