There is very likely not a reliable scientific guide to making and keeping friends, because it depends very much on the circumstances and the people involved.
Some people have many shallow friendships, others just a few deep ones.
Some people extend the hand of friendship early, others take years of persistence to become friends with.
Some people value time spent together on shared experiences, others value talking, others value gifts or help in times of crisis and others value being told how good a friend they are (once the friendship is clearly there).
Many people are friends with individuals they have known since high school and it is generally harder to make new friends after that point. It's also pretty easy to lose friends as you age due to differences of opinions or beliefs.
Perhaps the biggest skills in making and retaining friends is finding common activities and experiences you can share with them (outings, games, sport, dining, interests, topics of discussion, etc.) and finding the right time to invite them to engage with you in such things.
Very often there are logistical hurdles to overcome and that is all: do they have a car, is there a convenient bus, is the venue you are meeting nearby, are you available when they are, do they have a babysitter, is the suggested date suitable for them, have they currently got some other crisis or deadline to deal with, etc.
I find that if I ask to spend time with people too often, they may think I am "clingy" or "needy" and push me away. If I ask too little, the friendship never really gets established. It's a balance that is different for everyone.
To add to your list: Some people don't have any close friends and don't have any [diagnosed or suspected] neural atypicalcy.
Reflecting, I don't require people to have any obligation towards me, and if they don't want to spend time with me that's their call I'm not going to force myself on people, so when people just sort of drift past; that's just what happens. I feel like I could have friends if I was happy to play the part of someone who cared about TV celebrities, fawned over politicians, or idolised sportsmen.
Perhaps one has to be more assertive and demanding to some extent?
You can find friends doing volunteer work, taking courses that interest you, various meetings and groups. You may need to either find a group that draws you in, or try to genuinely talk to people and get to know them. If your goal isn't to be friendly, nothing much will happen.
Fwiw I do/have done those things. I have many acquaintances that will greet me in the street or whatever.
There focus here isn't me, and I'm trying to keep it that way: I mean to say that these things may help but shouldn't be viewed as sufficient.
There's something else needed, for sure. IMO only people who you spend time with outside of the original context are really friends - like someone you have as a colleague might become more than an acquaintance when you, say, go to a (non-work directed) event outside work together. There needs to be some sense of choosing to be with one another, not just happening to be in the same location because you both like volleyball, say.
Some people have many shallow friendships, others just a few deep ones.
Some people extend the hand of friendship early, others take years of persistence to become friends with.
Some people value time spent together on shared experiences, others value talking, others value gifts or help in times of crisis and others value being told how good a friend they are (once the friendship is clearly there).
Many people are friends with individuals they have known since high school and it is generally harder to make new friends after that point. It's also pretty easy to lose friends as you age due to differences of opinions or beliefs.
Perhaps the biggest skills in making and retaining friends is finding common activities and experiences you can share with them (outings, games, sport, dining, interests, topics of discussion, etc.) and finding the right time to invite them to engage with you in such things.
Very often there are logistical hurdles to overcome and that is all: do they have a car, is there a convenient bus, is the venue you are meeting nearby, are you available when they are, do they have a babysitter, is the suggested date suitable for them, have they currently got some other crisis or deadline to deal with, etc.
I find that if I ask to spend time with people too often, they may think I am "clingy" or "needy" and push me away. If I ask too little, the friendship never really gets established. It's a balance that is different for everyone.