g runs the command mu4e-view-go-to-url (found in mu4e-view-mode-map), which is
an interactive compiled Lisp function in
‘/usr/local/Cellar/mu/1.2.0/share/emacs/site-lisp/mu/mu4e/mu4e-view.el’.

It is bound to g, <menu-bar> <headers> <goto-url>.

(mu4e-view-go-to-url &optional MULTI)

Offer to go to url(s). If MULTI (prefix-argument) is nil, go to
a single one, otherwise, offer to go to a range of urls.

(defun mu4e-view-save-url (&optional multi)
  "Offer to save urls(s) to the kill-ring. If
MULTI (prefix-argument) is nil, save a single one, otherwise, offer
to save a range of URLs."
...

mu4e-view-save-url is an interactive compiled Lisp function in
‘/usr/local/Cellar/mu/1.2.0/share/emacs/site-lisp/mu/mu4e/mu4e-view.el’.

It is bound to k, <menu-bar> <headers> <save-url>.

(mu4e-view-save-url &optional MULTI)

Offer to save urls(s) to the kill-ring. If
MULTI (prefix-argument) is nil, save a single one, otherwise, offer
to save a range of URLs.

;; set Chromium path on macOS
(setq browse-url-chromium-program "/Applications/Chromium.app/Contents/MacOS/Chromium")

;; code to follow links in different browser
(defun browse-url-at-point-chromium (&optional ARG)
  (interactive)
  (let ((url (get-text-property (point) 'shr-url)))
    (if url
    (browse-url-chromium url ARG)
      (let ((url (browse-url-url-at-point)))
    (browse-url-chromium url ARG)))))

(define-key mu4e-view-mode-map (kbd "J") 'browse-url-at-point-chromium)