How do programmers know the codes to use when developing an app or website?

Answer 1

By learning and practicing, obviously.

Answer 2

When there's a feature a programmers want to add to an app or website, they'll think of a code that they can use to implement that feature...
For example you're developing an app using java and you want to display the current Date and Time. You'll think of the classes that deal with Date and / Time ,then import them into your code to implement that Current Date and Time feature.
https://code.sololearn.com/c6Ub4z2U2l0P/?ref=app