Basically, unless you setup a specific configuration, git remote updates
runs a git fetch
against all your remotes.
The man page states
update
Fetch updates for a named set of remotes in the repository as defined
by remotes.<group>. If a named group is not specified on the command
line, the configuration parameter remotes.default will be used; if
remotes.default is not defined, all remotes which do not have the
configuration parameter remote.<name>.skipDefaultUpdate set to true
will be updated. (See git-config[1]).
In your case, if you didn't define any remotes.<group>
nor a remotes.default
, and if your only remote is github, running git remote update
or git fetch
is completely equivalent.