Default remote for git fetch

Question

If I am on a local branch that is not tracking any remote branch and I give the command

git fetch

Given I have several remotes defined

Answer 1

If you have multiple remote repositories, and don't specify any remote repository name, origin will be used by default. If there is no remote repository named origin, then it will error out saying

fatal: No remote repository specified.  Please, specify either a URL or a
remote name from which new revisions should be fetched.

Additionally: How can I change this default remote without making the local branch tracking?

You can rename that repository name to 'origin' to make it default.

Careful: this won't work if the current branch already has an upstream specified on a different remote.
From git help fetch:

When no remote is specified, by default the origin remote will be used, unless there’s an upstream branch configured for the current branch.

In this case, you can change the upstream branches to use origin by editing the remote fields for branches configured in .git/config.