It's more confusing than it should be!
The US consulates around the world contract with third-party service providers to manage the visa appointment systems. Procedures vary by country and sometimes even by post within the same country.
The two major services providers are US Travel Docs and AIS.USVISA
Once you have decided where you will apply for your visa, you can look up the country you are applying in on the following website:
https://www.ustraveldocs.com/
If the hyperlink sends you to a US Travel Docs page, you can proceed to set up a profile, pay your visa fee and make an appointment there.
If the hyperlink sends you to the embassy's website, that country is not supported by US Travel Docs and you will need to look up the country you are applying in on the following website:
https://ais.usvisa-info.com/en-us/countries_list/niv
Once redirected to the corresponding AIS page, you can set up a profile, pay your visa fee and make an appointment there.
Warning: Do not use websites to book your visa appointment other than the above. They are likely to charge you extra and keep your information.