I can't find some apps in my Mac

There is a JW Library app available for the iPad, iPhone and iPod Touch, but there is no JW app available for download to the Mac. And there doesn't seem to be a way yet to download the iPad app onto the newest M1 Macs.

You can, however, launch Safari or another browser to visit the JW website (https://jw.org) and access the online library that way.

You also may want to consider upgrading the operating system on your Mac. Most 2012 models can be upgraded to Catalina. See: macOS Catalina is compatible with these computers - Apple Support and How to get old versions of macOS - Apple Support

Many apps for Macs are not in the Mac app store but can be dowloaded from the developer of the app. Be sure that you are downloading directly from the app developer and not from a 3rd party. As an example, if you wanted install the Chrome browser app, you would download it directly from Google (the developer of Chrome) at https://www.google.com/chrome/

What apps? Are you logged into your Apple-id account in which you purchased those apps? How did you install these last time?

You don't tell what Mac you have so no one can give you advice on that. What does it say if you go the upper left and select "About this computer" in the Apple menu?