Once you've downloaded the STEX Collection Vol 1, 2, 3, extract the archive files using a tool like WinRAR or 7-Zip. The archive files will likely be in a .zip or .rar format.
You may see "free install" or "free download" associated with these collections in various places, but here is the official status: stex collection vol 1 2 3 for simcity 4 free install
The vast majority of high-quality SimCity 4 mods are hosted on . You cannot download files without an account. Registration is free. Once you've downloaded the STEX Collection Vol 1,