If you get this error "unable to locate java run time on mac to install fabric .jar on mac" when running the fabric setup file, go to

 https://adoptium.net/temurin/releases?version=21&os=any&arch=any

and select the download the JDK for your correct system. 

If you are on a mac, click on the apple logo on top left and "about this mac"
Download the aarch64 if you are on an M chip mac 
(it would say "M1 or M2 or M3")
If it says "Intel or Intel x64" get the x64 version. 

First run the fabric .jar file to install it for client, not server. 
When its done you can close it and go to your Minecraft Launcher and ensure you have selected the fabric instance. 
Go to instance settings for fabric and click on the folder icon to open the folder that has your fabric installation. 
Drag and drop the mods folder anywhere in that opened dir, and it should move all the files into the mods folder for fabric. 
Do not run any .jar files in the mods folder
Then, you are done and can join the server.