Just like with everyother layer, TCP segments have a generic data section to them. This payload section is actually the entire contents of whatever data applications wants to send to each other.
There are a lot of protocols used at the applications layer, and they are numerous and diverse, it wouldn’t make sense for us to cover all of them. Even so, one concept you an take away about application layer protocols is that there’s still standardized across application types.
There are lots of different web browsers. You can be using Chrome, Safari, etc.. they’ll need to speak the protocol. Same thing is true for web servers. In this case, the web browser would be the client and the web server would be the server. The most popular web servers are Microsoft IS, Apache, NGINX, but they also need to all speak the same protocol.
For web traffic, the application layer protocol is known as HTTP.
All of these different web browsers and web servers have to communicate using the same HTTP protocol specification in order to ensure interoperability. The same is true for most other classes of application.