These are compressed formats, just as pc Windows uses .zip. Dmg is a disk image & will open with Apple's Disk Copy (double-clicking dmg should open Disk Copy by default, which will then open the disk image onto your desktop. Your apps/installers will be inside it.) Sit files will open with StuffIt Expander. You'll need to drag & drop the .sit files onto Expander (unless you have StuffIt Deluxe where you can use a keyb'd shortcut.) Most applications will use either a .sit/.dmg extension. LW search for these to find what apps you want, it will list as many it can find during that search. Choose your file type search to find what you want. For specific items, type in the name of what you want. Whilst I'd like to, I can't tell you about serials on this forum.
You can dwnld from any connection from modem up to T1.