It's very rudimentary, but I want to make an inquiry form
I was watching this video, but I'm stumbling because I don't know how to upload php.
Even if I try various things and search, only the upload method using php comes out and I am in trouble.

The server is Star Server's free PHP + MySQL, and the domain is paid.
The FTP software uses FileZilla.
I would be grateful if you could teach me how to upload php as easily as possible.