Looks are deceiving.

It is possible to create a website or app with one person, but there is a limit to the complexity.

For example, just glancing at the Indeed website:

• They need engineers to maintain their codebase. For a site like this, it might easily be over a million lines of code. (There is a lot of functionality you can't see, like internal auditing and management systems, in order to prevent legal problems -- see below.)
• They need engineers to maintain their servers.
• They need people to write content in the Career Guide.
• They need database administrators to tune, backup and monitor their databases -- often geo-replicated around the world.
• They need security engineers to prevent hackers from taking all this valuable personal data they have collected.
• They need employees to "answer the phone" when their customers have problems.
• They need employees to work with companies and the schools to find out their needs.
• They need lots of people to keep spam out of their various databases (schools, jobs, training, resumes, etc)
• They may need employees to verify companies aren't scamming personal data.
• They need HR, Finance, Legal, etc. Legal is probably especially big, since they need to know the hiring laws in every country and every state. (Did you know it's perfectly legal to age discriminate in most states, as long as it's not against someone over 40?)