If your mac is the older version yes you have it can go as high as 16GB the newest version of mac book has the right built in on the mother board also they no longer carry hard drives they come with a special design of SSD Drives
Yes you can change your RAM on all Macbooks. You would need to look up for model macbook on the Apple website and look for the manually because each macbook is different.
Depends which macbook you've got. Mine is a mid 2012 model, that I upgraded from 4gb to 8gb. Running very smoothly in combination with my SSD upgrade as well. Look up YouTube tutorials if you're not sure how to change your RAM. Cheers.