I'm not really an FA, but I frequent your sub because i once had the same issues you all have many years ago. I see a lot of advice from outsiders get shut down so I've hesitated to comment. But I had these exact questions a few years back so I'll try my best to explain how I dealt with it.
Never had friends and thus i never was invited to parties or events. Even when i talk with people they lose interest in the conversation pretty fast and end it. How could i develop social skills in this situation?
There's no easy way to develop social skills other than to practise. Books and videos on how to make interesting conversation are great starting blocks, but they were not game changers for me. I had to put myself out there, force myself into uncomfortable scenarios, and learn from failure. And if you're anything like me, there will be a lot of failure. But if you want to develop social skills, you just have to persevere.
I wouldn't recommend diving into the deep end, that's just begging for punishment. I slowly worked my way there by trying to spend more time with the few people I was comfortable with. For me, this meant primarily my family and the people I played online gaming with. Actually, a lot of my early development came from building on friendships with my online friends. If you're going down this path, try using voice chat as much as possible so you can practise actually vocalising your thoughts. After you feel comfortable in any particular group, you expand your social circle.
It is extremely important to constantly be putting yourself in uncomfortable situations so your mind just adapts to the anxiousness. And be patient! This is a slow process that might never stop for people like you and I.
Normally i wear what is comfortable to me. I don't understand fashion and what is ok to wear, what colour goes with this or that. Never understood how people can do it and be so obsessed about it.
Eh, people overemphasise fashion. When I'm not dressed professionally, I am primarily dressed in "athleisure" primarily because I don't care about fashion and heavily prioritise comfort. Dress how you want to dress, and what you think makes you look good. That's the kicker, you still have to put in effort to look good, but that doesn't mean you have to follow fashion trends. Follow youtubers such as "Teachingmensfashion" to get ideas to develop your own style.
That being said, all this is conditioned on my next point. Fashion isn't so important if you have an impressive physique. If you don't have this physique, or you are still working towards it, then maybe fashion is more important. I can't really advise on this specific point as I skipped any fashion considerations in lieu of building muscle as fast as possible.
How to exercise and eat healthy?
I can't stress enough the benefits of getting fit. It'll improve how people will perceive you, which will give you some validation and help bolster your confidence, which will then feed into your willingness to push yourself into uncomfortable scenarios, which will help develop your social skills further. It will also teach you about discipline, dedication, motivation, and sacrifice.
Get yourself a gym membership and go. This is purely a self discipline issue because rarely are people ever too busy to not commit an hour a day to exercise. I struggled with my self discipline and I had a very on/off relationship with the gym at first that lasted half a decade. Most people will always lack the motivation to go to the gym, that's why most people don't have athletic builds. But the more you go, the more it'll be ingrained into your routine, the easier the commitment will become.
Everything you'll ever need to know in regards to the gym (workout routines, diet, etc) can be easily found on the internet. Just find a cookie-cutter plan that fits your body shape and commit to it. Remember, at the start, you're trying to change your mentality. After you have the basics down, then you can work on the little nuances of body building.
PS: I acknowledge that there is no "gym for your face". As a guy who is short, with a receding hairline, and an extremely weak chin/jaw, I really do acknowledge this fact. That being said, why dwell over things that aren't in your control and let it affect what is in your control?
How do people manage all that? Seriously how can people excel at these life aspects?
The "fashion" and "physique" aspects are easy, it's merely a matter of putting in the effort. No one ever has an excuse (special circumstances exempted such as a medical condition) to not dress well or have an athletic body. If you don't have these, that's your choice and you have no one to blame but yourself.
The "social skills" aspect is definitely the hardest to improve. There's no magic pill, you just have to try, fail, and learn over and over again.