how do I customize spacemacs default buffer

I also wanted to change the startup buffer (or spacemacs-buffer as it is called) and I found https://www.reddit.com/r/spacemacs/comments/55p1td/dont_like_the_new_footer_or_customizing_your/ where basically says that the design of the page is done in spacemacs/core/core-spacemacs-buffer.el (https://github.com/syl20bnr/spacemacs/blob/master/core/core-spacemacs-buffer.el). And I did so, I didn't want to see the release-notes (they are meaningles because now all the development is done in the development branch) and I wrote

(defun spacemacs-buffer//notes-insert-release-note ())

in my .spacemacs file in section dotspacemacs/user-config

It seems like you're looking for something more along the lines of Workgroups2, a session manager for emacs that saves and restores open buffers along with thier locations and sizes. Also supports multiple workspaces.

Comprehensive info is available at readthedocs: Workgroups2 - Emacs Session Manager

And git: Workgroups for Emacs

• Emacs works differently than other editors you may have used in the past and Spacemacs is really "just" a configuration of layers and extensions that make Emacs more accessible to new users. A key difference, as Dan points out, is that everything begins in a buffer... and is only later written to a file type.
• For instance, having the Scratch buffer always available is very handy. One can immediately enter anything here (an org TODO, a Clojure script, a LISP fragment).
• It only becomes something else when you do something to the buffer; that is, when you save it to a file (foo.org, foo.clj, foo.lisp, etc). Saving a buffer as something changes its nature from general to specific, from potentiality to actuality.

  This has interesting philosophical considerations!

Use emacs-dashboard ? It is

A shameless extraction of Spacemac's startup screen

that

• Displays an awesome Emacs banner!
• Recent files
• Bookmarks list
• Recent projectile projects list (Depends on projectile package)

they're working to make it easily customisable, AFAIK.