You have probably heard that Mac OS Catalina, 10.15, will end support for 32-bit apps. Sounds geeky, and it is in some ways, but here’s the bottom line: some of your applications will quit working. Period. End of story. 

Here’s what Apple says:
https://support.apple.com/en-us/HT208436

So… what to do?

1) find out which apps you have that will quit working in the next 60 days if you upgrade to Catalina, when it comes out.
Here’s the easiest way: this free app:
https://www.stclairsoft.com/Go64/index.html
(there are more less convenient ways to find your 32-bit apps, using Apple software) “
https://www.macrumors.com/guide/32-bit-mac-apps/

then you can
2) simply not upgrade to Catalina (or put them all on another computer that will never be ugraded.)
3) wait for the app developers to republish their software compiled for 64-bit
4) toss them out and find a suitable replacement
5) run El Capitan (10.11) in a virtual machine. (That’s my choice.)

Virtual machines

If you have Parallels or VMWare (probably to run Windows) you can find a copy of 10.11* and install it, to run it virtually. Then all you need to do is move your 32-bit apps to it, and away you go: you can upgrade to Catalina, but keep El Capitan around inside your VM.

https://www.macstrategy.com/article.php?148

*You can actually choose any OS prior to Catalina to virtualize. I went with El Capitan since there are trusted opinions that it’s the most suitable. Obviously, if you have some 32-bit app that requires, say, Sierra, then that OS would be your choice.

It has been my personal experience that Parallels works surprisingly well in general.

hth