Science of the hadith[s]

Hello you guys,

I know this is probably not the right spot, but I was unsure where else to put this!

So my question is to those who have knwoledge on the hadiths. I have read 4 of the 6 major collections of the sunni hadiths, but have never gotten any knwoledge on isnad, or anything about the "science of the hadith", and appearently there are a lot more hadith then just these 4 compilations (and the 6 total sunni ones.)

So, my questions are as follows..

1. What is the oldest collection of hadiths, and when were they written? Are they still avalible today? Are they widely circulated?

2a. How does one select what hadith is more authentic then the other?
2b. If it is by isnad, how can one verify the isnad? And can you elaborate a bit more on the isnad?

3. Do the shia use the same hadith as the sunni? I know that the general answer is no. So do they use any hadiths similar to the sunni?

4. Were there any major hadith collections before bukhari? Or did bukhari basically compile all those former hadiths into one?

thanks!