Answer 1

css3 is technology,
bootstrap is a framework, based on js and css3, that gives you ready made CSS3 rules for classes, or widgets and functions made in js for classes.
css3 means 3rd version of CSS, css3 is technology just as JavaScript is.
bootstrap makes it easy to make your page responsive, get a nice ready made theme for your webpage, and many inbuilt widgets, you don't need to make on your own.

which one is better to use? pure css or bootstrap framework?
it depends, both has its own advantages and disadvantages, if you make your own CSS code , it will be short, of low size, will be easy to load, will take less amount of memory when opened, no extra imports and will be customized, on the other hand , will take much time in development, will have to make everything from scratch, manage all the classes you made.
but in using framework, you can develop fast, it makes easy to manage the project, will be using optimized algorithms and rules.