Yes, follow the documentation I linked. You didn’t configure the Ondrej repositories, so you shouldn’t expect to have the php7.3-* packages available to install. That’s one command: LC_ALL=C.UTF-8 add-apt-repository -y ppa:ondrej/php && apt-get update
For the repo errors…reading it more carefully (once again, I have a hard time with the screenshots, instead of copy/paste, it’s hard for me to read the screenshot), I see it’s just that you have repeated entries in your sources.list. You shouldn’t do that, and should fix it, but it’s also harmless.