Presuming you've already installed olivetti and exotica-theme, I would write a function that does steps 1, 2, and 3, and then call it towards the end of your initialization file.
(defun my/initial-layout ()
"Create my initial screen layout."
;; 1. to have exotica theme mode enabled, along with
(load-theme 'exotica t)
;; 2. having org-mode launch in scratch buffer from the beginning, and
;; 3. to have olivetti mode enabled too.
Since it's an
(interactive) function, you can M-x my/initial-layout at any time to return your screen to this state. You may have noticed that I added
(delete-other-windows) and commented them out. It's beyond what you asked for, but I think you'll like these too.
org-indent-mode makes org documents look better, and the
delete-other-windows is to make sure that you're looking at the scratch buffer and nothing else.
PS: You seem new to Elisp. I recommend using M-x ielm to start an Elisp REPL while experimenting with this language. Also use C-h f and C-h v to ask Emacs for documenation on various functions and variables. emacsdocs.org is also a good place to find documentation.