Facebook is currently testing putting the messenger chat back into the main Facebook app. Jane Manchun Wong, an app researcher discovered this and realized that Facebook and Facebook messenger could be accessed through one app. Clicking on the Chat icon will direct you to a separate section in the app instead of redirecting you to a whole other app.
Messenger was created separately in 2011, as its own app separate from Facebook. Facebook removed messaging all together from the app in 2014. Now in 2019, they are testing out combining them again.
Wong explains that even though you will be able to access messenger in the main Facebook app, you are limited to only sending and receiving messages. You will have to go to the separate Messenger app to get the full functionality of Messenger.
Facebook still intends to get full use out of both apps. Wong believes that each app is intended for different audiences, and brings in different types of people. It is still unknown when this integration will officially happen.