As other people have said it’s somewhat all of the above, though the biggest and most visible on xray is the narrowing of the joint space from deterioration of the cartilage. This causes arthritis, and also calcification of the joint which is also all worsened with age and varying bone density which can change based on things like ancestry, gender, habits (drug use/smoking) or medical conditions.
Edit: I also forgot to mention that occupation can make a big difference here. Being overweight AND having a job that is strenuous on the knees (construction/manual labor/heavy lifting) is a bad combo which can lead to joint deterioration and premature joint replacement as well.
I don’t usually donate to charities because the vast majority of the donation never actually goes to the cause it was donated for in the first place. However, I try to give cash to the homeless as often as Ican. On my drive home I frequently see homeless people and I try to give money and also bottled water that I usually have in my car. I don’t know what they’ll use it for, but it’s something I can do right there and then for them and I know they at least have the opportunity to buy some food / necessities, and I know they have water.