It has to do with what protocol they are using to communicate with the server/cloud/whatever. If they are using HTTP/S (hyper text transfer protocol) then it is

It has to do with what protocol they are using to communicate with the server/cloud/whatever. If they are using HTTP/S (hyper text transfer protocol) then it is web/www. Otherwise, it is some other protocol. For example, email (technically) uses SMTP (simple message transfer protocol). However, all of these run on top of the Internet Protocol (IP). So, everything is internet, but not everything on the internet is http. Apps are almost certainly using HTTP under the hood to communicate with their servers, so technically they are a part of the web, which is a part of the internet.