32bit vs 64bit

To add—Locate all 32-bit apps in System Info

http://osxdaily.com/2018/03/06/find-32-bit-apps-mac/

The popup is a generic warning only. You have to do nothing, 32-bit apps continue to work on High Sierra and Mojave.

In the future release of macOS 10.15, applications will have to be 64-bit. App developers will issue updated apps if they want their apps to work on these future releases.

32-bit app compatibility with macOS High Sierra 10.13 ... - Apple Support

https://support.apple.com/en-us/HT208436

Only apps, command tools, code bundles, and anything else containing executable code need to be supported by 64-bit processor architectures. File types typically are not effected unless they are dependent on the QuickTime 7 framework like Final Cut Pro, then there are some necessary conversion.

A 64-bit program typically is little faster. If your application is memory hungry like Photoshop, 3D rendering, etc. having a 64-bit version will give it access to all the computer's memory. Otherwise it will be limited to somewhere around 2GB/ application, so two applications can access a total of 4GB.

There is no difference between 64-bit and 32-bit applications stability.

No. It will be up to the developers to upgrade their apps or offer a new version that's 64 big. Some may update them, others will offer a new version at a cost while others will abandon their app at this point.

There's a free app, Go64 that will scan and show you all 64 bit apps or 32 bit apps and has a button to take you to each apps website so you can check to see if there's a 64 bit version available or will be available.

No. The Apple apps will, or they will go away. You will have to update third-party apps.