This is all good info except for the gender thing. The round/long difference is just a growth habit. Watermelon plants (and other cucurbits like squash, zucchini, cantaloupe, etc) produce male and female flowers. Only the female flowers produce fruit and must be pollinated by a male flower to do so.
I spent a season working in a packing house for watermelons. They'd come in by the crateload and we were allowed to just grab one to eat any time we wanted.
The trick I was taught, and which proved to be pretty reliable over the course of the season, was to feel the veins. (This is possibly what's being described as webbing here?) Watermelons aren't smooth, they have wide "veins" running top to bottom and you can feel them if you put your hand flat on the side of the melon. The bigger/poofier/wider the veins, the more ripe is it.
I'll just remember "pick the most fucked up looking melon with patchy orange spots and ugly crisscross webbing". It's probably not going to make the photo reel but it'll taste good.
None of these visual methods are reliable as these things differ greatly amongst melon varieties. The easiest way is just to knock on the watermelon like you would a door, if it sounds hollow on the inside, then it's ripe.
Why the green arrow for the "wrong" one, and the red arrow for the "right" one?
3 of the 4 items (gender doesn't matter, variety does) are generally correct.
Source: I'm a former watermelon "cutter" (the guy that goes out in the field first thing in the morning and cuts the good melons off the vine, and turns them belly side up so it's obvious to the field workers which melons to load up)
Also, with the whole thumping thing, most people just look silly doing as they don't know what they're doing. If you do thump, ones that have a higher pitched ping are still green, and that have a really dull/flat thud are over-ripe/too gritty/sugary. Also, weight should feel right, too light and it's overripe/rotten.
In general, any melon sold at the store should be good, just take one and stop trying to be a hero. At least the farmers I dealt with are pretty ethical, they aren't purposely shipping bad melons. It just takes experience of seeing/handling melons for a while to get the "picking one" correct. Most store I know of have a satisfaction guarantee anyway, take pictures if it's bad and when you go back get a refund if you're that concerned with it.
Morgan Freeman taught me to flick melons to determine the ripeness in Unleashed, and I have honed the skill to a masterful level. Get weird looks from strangers sometimes tho..
I worked at a grocery store and the old timer who trained me in the produce section said that all you needed to find the best watermelon was learn how to give it a slap and if it sounds like a basketball that's a good watermelon. Learning the slap takes some practice but when you do it right, you'll know.
Man, coulda used this a few days ago when I got a watermelon for the fam - I remembered the color of the spot part, but had forgotten the other things to look for 🤦🏽♀️🤷🏻♀️ the one we got was decent, but coulda been more flavorful imo