emacs-startup-hook is a variable defined in startup.el.gz.

Value
(emacs-startup-hook&init-after-load-functions
 emacs-startup-hook&which-key-mode
 emacs-startup-hook&recentf-mode
 emacs-startup-hook&evil-mode
 emacs-startup-hook&restore-startup-values
 emacs-startup-hook&gcmh-mode)

View as literal Set

Documentation
Normal hook run after loading init files and handling the command line.

View in manual

References
References in startup.el.gz:
(defvar emacs-startup-hook ...) 1 reference

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Debugging
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Source Code
;; Defined in /usr/share/emacs/29.4/lisp/startup.el.gz
(defvar emacs-startup-hook nil
  "Normal hook run after loading init files and handling the command line.")

Symbol Properties
hook--depth-alist
  depth-alist
variable-documentation
  ("startup.elc" . 10555)