As a developer, you can spot bad code by looking for a few key things. First, you can look for code that is hard to read or understand. This could include code that is overly complex, lacks clear structure, or is poorly documented. Second, you can look for code that is inefficient or that does not follow best practices. This could include code that uses unnecessary or outdated algorithms, or code that does not take advantage of modern programming languages and frameworks. Finally, you can look for code that is not modular or reusable, or that is prone to bugs or other issues. By being vigilant and keeping an eye out for these things, you can help identify and improve bad code.

You can't spot a bad code until you run it.
What is the use of beauty if it doesn't run bug free?

What you listed above are ways to spot a bad design and in some cases how to differentiate who is a junior developer.

Choice of algorithms, age of the language, choice of frameworks are just personal preferences and job requirement dependant. Nothing more.

Infact I am currently building an ecommerce site for a US a company with only pure PHP, HTML, Mysql, and CSS. The company strictly forbids me using any framework and Javascript is not allowed anywhere too on the site.

It should run on onion site too. And must function perfectly like high popular ecommerce sites like amazon, Ebay and co.

While reusability or modular codes is matter of personal style sometimes and how big the project is.