First, try launching the scan software from your PC. This requires you to install the full driver and software package (from the CD or https://www.usa.canon.com/internet/portal/us/home/support/details/printers/inkjet-multifunction/mg-series-inkjet/mg7520/pixma-mg7520?cm_sp=CSO-_-PFListing-_-MG7520
). Initiate a scan from the computer to find the settings that work best for your usual documents. Note: the full software needs to be installed before you connect the all-in-one to the computer if you use the USB port.
Next, make sure that the scan monitor program is running. For Windows, there are two programs involved the Canon Scan Gear (driver) and the Canon IJ Scan Utility. For the all-in-one on the network, it's the IJ Scan Utility EX. (The program names may be different under the Mac OS.)
It's the IJ Scan Utility that you need to use to set the response from the buttons on the MG7520. Launch the utility and click on the Settings button (bottom right). Click on the Operations icon (the box with the arrow pointing out towards you). Set the default settings for each of the scan buttons (Save to PC (Auto), Save to PC (Photo), etc.) You need to set the folder to save the information and resolution. You can also set the software that you want to use to manipulate scanned data if you plan to crop the image or deal with a skewed image. Click OK to save the settings.
Now, you should be able to use the Operation panel buttons to launch a scan when IJ Scan Utility is running.
I hope this helps. Screen captures of the IJ Scan Utility Settings page are available in the manual. See the section starting on page 793 of the PDF. The PDF and an EXE file with the manual available here: https://www.usa.canon.com/internet/portal/us/home/support/details/printers/inkjet-multifunction/mg-series-inkjet/mg7520/pixma-mg7520?tab=manuals