Contents of disk can't be changed...

What app are you trying to install and where are you trying to install the app?

Are you installing the most recent version of the app? If not, then the older app installer may not be aware of the newer security settings of macOS and may be attempting to install to a restricted area.

You should never have to disable System Integrity Protection (SIP) in order to install any legitimate app. Disabling SIP is extremely dangerous to macOS. Even disabling SIP on macOS 10.15+ will not allow you to write to the macOS system protected volume anymore. Here is an Apple article regarding the read-only macOS system volume:

About the read-only system volume in macOS Catalina - Apple Support

For anyone who says it is their Mac and they can do as they wish, then I have to say that you are using the wrong OS since Apple now says users should not be able to modify the macOS 10.13+ system volume.

Yes, but you need to use "csrutil enable" from Recovery Mode according to the Apple documentation.

If an app has an installer, then the app is supposed to respect the default security settings of the OS. While SIP has existed since macOS 10.11 in 2015, it was only with macOS 10.15 in 2019 that Apple made the macOS system volume read-only so it is not surprising that a developer would take the easy way out and have users degrade the security of their OS instead of modifying their app to respect the default settings of the OS. I'm guessing there is a newer version of this app which will work correctly with Catalina.

As for the Unity3D app, you probably will need to contact the developer or ask on the forums dedicated to the app to see if they support running the older 2018 version on Catalina (I doubt it). If you want to use the older version of the app you can either try installing it on macOS Mojave if your Mac supports Mojave or possibly use a Virtual Machine to run Mojave (it will run slower).