how does web-hosting sites host websites in backend like heroku, go-daddy etc?

i want to know by CODING(i dont need all codes but a reference like use this then that etc.) means how the coding part works in web hosting sites like heroku, godaddy etc?

Suppose i want to host a php ,node or django website , for that language how will the server behaves or detects the php code and host it online?

Any references would help me.

Thanks in advance