1. The apk installers are in a system partition inaccessible to users who aren't rooted (admin access). You can download an app called 'App Backup and Restore' to make copies of these apk files and dump them in a folder of your choosing. Once you have them backup up, you can navigate to that folder from your PC.
2. Treat the apk's as installers. Basically they are analogous to .exe files in Windows for installing programs. Basically, put them on your SD card, open them using a file manager app on your phone and they will install. Or you can put them in the App Backup folder, and they will be detected by that app for install. Remember you have to enable non-Market installation for this to work.