Can i download a portable app to my computer harddrive and a usb device but only install it on the usb drive

If I understand your question correctly, all of the officially released portable apps take their settings with them on whatever drive they are installed on, whether it is a hard drive or flash drive. I hope this helps.

can download a portable app to my computer hard drive and usb device, but only install it on my usb device

Yes, you can download the installers to your computer's hard drive (and copy to your USB device) and then re-install as required.

All the officially released apps should be on the Portable App Directory page
https://portableapps.com/apps

Please note: some apps use an Online Installer where the Portable App installer has to download the programs installer. eg: Google Chrome Portable (https://portableapps.com/apps/internet/google_chrome_portable)

so if i have to reinstall it on the usb drive i have the master app on the pc, copy it to the usb drive and reinstall it

This means that you won't be able to reinstall those programs unless you have internet access.

But most of the Portable Apps installers have the program's installer in them.

I hope this answer helps