Unsure about some concepts in genetic library construction

I'm a little confused about some concepts in genomic and cDNA libraries. First off, I'm still unsure whether or not a phage vector can be used to build a cDNA library. Also, if you're building a genomic library, and you ultimately have to use a biotinylated probe to find your gene of interest, what would you still need to know about that gene? Do they make probes before they know the entire gene sequence, and still have more to examine? I also haven't heard of PCR being used at all for these libraries, so I'm wondering why that isn't a more viable way to amplify the genes of interest within the library, or if it is, but only later on. I guess I understand the uses for cDNA libraries, but just don't understand why a genomic library is so useful for amplifying (though I'm undoubtedly sure that it is).