where are all my photos that were on the old iPhoto app?

Did you migrate the old Library?

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point it at your old iPhoto Library. Photos will then convert it for you.

Assuming you transferred your library to the new computer, you have to transition to Photos.

Launch Photos - the program with the icon like this:

with the option key held down. From the dialog, choose your iPhoto Library. Photos will make a new library from your iPhoto Media. Your iPhoto library will not be changed, though it will be renamed.