Nostr has no governing body. Community driven development.
Of the 3 apps Amber is the most mature, and is made by nostr:nprofile1qqs827g8dkd07zjvlhh60csytujgd3l9mz7x807xk3fewge7rwlukxgpz9mhxue69uhkummnw3ezumrpdejz772u5wm
If you are on Android this is likely the best choice overall.
Nowser is made by nostr:nprofile1qqszjvsfwh0c2hlrffa5ttdzgg0zcaquxlqpx6gplerhzvafr6cckpcppemhxue69uhkummn9ekx7mp0qy2hwumn8ghj7un9d3shjtnyv9kh2uewd9hj7qg4waehxw309ahx7um5wghx77r5wghxgetk9u0ww7wd and includes a web browser, which is handy for Nostr web apps