MacBook Pro (Retina, 15-inch, Mid 2015) macOS majove 12.14.6
my MacBook Pro (Retina, 15-inch, Mid 2015) have to upgrade to catalina, If I upgrade, what about last installed application. have I reinstall all of them?
I understand you are considering upgrading macOS and are concerned about the possibility of having to reinstall all your existing applications.
The short answer is no, but the longer answer is that really old, 32-bit applications will no longer run at all and you would need to purchase new and upgraded versions of them, or find suitable alternatives. This appears to be a primary concern to users of Microsoft products, but any products of similar vintage may be affected.
This is what I do, and what I recommend: Upgrading macOS without fear - Apple Community. But as long as you read and follow Apple's macOS upgrade instructions (which include the necessity of creating a backup) you have nothing to be concerned about.