How do I find files added/modified by app installation?

I am not sure if this the type of information you are looking for ( I think you want to see what the actual Deb package is doing it, and for that you need to check with each particular developer) but this is a start.

Recent actions by Cydia are in

Code:

/var/tmp/cydia.log 

Only show the last time you were on.

A full list of installed apps can be found in

Code:

/var/lib/dpkg/status.

If you want a shorter list, you can start a terminal and type:

Code:

dpkg --get-selections

If you organize the contents of the folder /var/lib/dpkg/info by "last changed" time, you can see from the .list files when the packages were last upgraded/installed