There are default apps that open certain types of files in macOS Mojave. However, you can change this. To open a file once with a specific app, do one of the following:
Drag the file onto the app icon in the Finder or the Dock.
Select the file in the Finder, choose File > Open With, then choose an app.
Control-click the file, choose Open With, then choose an app.
Open the app, then choose File > Open.
To permanently change the app used to open a file
In the Finder, select the file, then choose File > Get Info.
You can also Control-click the file, then choose Get Info.
In the Info window, click the disclosure triangle next to “Open with.”
Click the pop-up menu, then choose the app.
To open all files of this type with this app, click Change All.