(1. why?) Well it does save cash :) I only consider it recreation when I have friends over. The main reason, however, because it's normal. Probably a cultural thing (not judging!). In the Netherlands (or at least in my circles), it'd be considered "weird" if you eat out (or takeout) more than once or twice a week. It's just considered normal that people cook, like doing laundry. Also means it's not going to be fancy always, sometimes it's just fuel and nutrients (some/any combo of vegetables, proteins and carbs). I also believe it's more expensive to eat out than in the US, relative to cooking from basic ingredients (or maybe vegetables are cheaper, I dunno). I can make tasty dinner for about €5-7 (two portions). Getting something healthy readymade for you is at least €10 (one portion).

(2. planning) Remember that article a while back about "people sized cities" (or something like that)? In NL, if you live in a medium-size town, a supermarket is never farther away than a 5 minute bicycle ride. My closest one is less than 200m, the next (bigger) one is 700m. But I'm lucky :) There are people that do weekly shopping of course, usually either big families or people that live out in the country, where the nearest supermarket is a cardrive away (like in the US).
Besides shopping-planning, if I cook for myself, I usually make at least two-three portions because it's really the same amount of work as making one. Also you can't buy many kinds of fresh ingredients in single-meal amounts. So I do need to plan ahead to be able to eat all that stuff before it goes bad. Sometimes that means having weird dinner-type things for breakfast. I hate throwing food away, but I try not to be too hard on myself if it means I ate healthy stuff, but messed up on the planning.

(3. what do I cook) Everything. My staple go-to meals, however are some kind of pasta-vegetable-salad (fusili pasta, cherry tomatoes, any kind of salad vegetables, cucumber, paprika, shallots, etc, any cheese, salami, bacon, eggs, olives etc whatever I have in the fridge + dressing from mayo+mustard+honey) and broccoli + fried tempeh or organic sausages. Also, brussel sprouts. These last two vegetables seem (for me) to cover most nutrients I've been missing when I feel I haven't been eating healthy. At least I feel better immediately :)

I also love cooking weird and/or special stuff. I could write a whole series of posts on that. And I did, but the stories/pictures are spread over many different parts and ages of the Internet.

(4. diet) Not really. I did paleo/keto for a while, twice, it was fun, tasty and really interesting to find your body can go into a different "mode" (ketosis) for energy. However I found that my physical condition for running improved a great deal when I added more carbs to my diet. I probably did something wrong, cause many people seem to be able to do sports just fine on keto diets. Anyway, these days I just don't eat a lot of carbs. I find I have to pay attention to eating enough (sorry I'm probably weird), both calorie-wise and nutrients (as I said, broccoli/sprouts).

(5. hacks and tips) I have zillions. Get a good knife. Learn how to use your microwave effectively (it cooks broccoli and many other things to perfection, if you know how--I'll explain if asked). Dishwashing machine. Check out the kitchen-tools section in shops every now and then, see if there's any tools that may make your particular way of cooking easier. If you don't have bbq tongs, get them, even if you're vegetarian. Fresh ginger keeps for a long time in the freezer. Get to know the chemistry behind cooking, preparing food, nutrition, etc. Watch Alton Brown's series Good Eats, it'll not only teach you loads of basic cooking techniques, but also the how & why behind these techniques, and even the chemistry/science of it. Plus it's entertaining. Just pick some random episodes on topics you think are interesting. I guarantee you'll learn something useful from every one of them. Many can be found on YouTube.