I could be considered “tech savvy”, I know a bit of C/PHP and a lot of shell script. Explain ro me how I could learn that without a computer (I’m also self-taught)

By using the computer or phone at home. Roughly half of the programmer workforce currently alive went through childhood without a mobile phone, because they didn't exist for regular consumers. And personal laptops for children would've been perhaps an option for the top 1%, but probably not even them. Since you just didn't have electronics in school.