It is very broad , depends on the project, it depends on the need , you can not say.
It may be that if the project is simple only bootstrap.min.css will be necessary, if it is a project that will use popups (bootstrap apis) then it would require bootstrap.min.css and jQuery and bootstrap.min.js , , for example in most projects I use only bootstrap.min.css , but it has projects that use .js and .css
If it is a project that needs to be debugged, then adding the .map together with the non-minified sources ( .min.js and .min.css ) can help a lot to understand some behavior, generally I would use only in development environment or homologation .
About the map: link
I have projects that do not even need jQuery, it is usually so little use that I just take advantage of the same GRID system.
What are bootstrap-grid, bootstrap-reboot and bootstrap-bundle
You probably downloaded the source for development, this 3 together is the bootstrap, if you do not want to participate as a Bootstrap developer or create a fork of it this will be interesting (download the source with the separate files), similar to this project link , which is a fork
But for most projects that you get isolated, you do not even need to, because we end up needing one or another hour of a specific functionality, unless you only need one or other Bootstrap functionality, so picking up only bits of it useful in most projects using CDN will be more than enough.
If you only need CSS add:
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css" integrity="sha384-PsH8R72JQ3SOdhVi3uxftmaW6Vc51MKb0q5P2rRUpPvrszuE4W1povHYgTpBfshb" crossorigin="anonymous">
If you need the APIs add:
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css" integrity="sha384-PsH8R72JQ3SOdhVi3uxftmaW6Vc51MKb0q5P2rRUpPvrszuE4W1povHYgTpBfshb" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js"integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js"integrity="sha384-vFJXuSJphROIrBnz7yo7oB41mKfc8JzQZiCq4NCceLEaO4IHwicKwpJf9c9IpFgh" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js"integrity="sha384-alpBpkh1PFOepccYVYDB4do5UnbKysX5WZXm3XxPqe5iKTfUKjNkCk9SaVuEZflJ" crossorigin="anonymous"></script>
Customizing Bootstrap
Note that until 3.3 it was possible to customize the bootstrap to download only the desired components link , however it seems that this is not yet available in 4.0, but of course it is possible to "compile" manually, downloading the sources, just as you did
