I want to collect files from the git repository on Backlog in bulk.

I thought you just wanted to do git clone.

If you have access to the repository, you can use the HTTPS URL to do git clone on the console.

Example:

 git clone https://nulab.backlog.jp/git/BLG/tutorial.git#← Rewrite the URL as appropriate

If you are using a GUI client for git, you can clone it using the clone function of that client.

Also include a link to the tutorial provided by Backlog: Clone a remote repository.
Select your environment to see examples of consoles and GUI clients.

Perhaps you would like to retrieve the contents of the repository in zip files, such as the Release feature on GitHub.

The Backlog Help Center has also been asked to do so, but the fact that the help has not yet been clearly stated in the help has probably not been implemented.

I want the ability to download the Git repository|Backlog Help Center

An alternative is to use git archive.If you have already cloned locally, you can create an archive with the command.

Make the contents of the Git repository a zip file - Qiita

$git archive HEAD --output=/path/to/repo.zip

Alternatively, it seems that there is a way to archive the latest information without cloning it.

Get only specific directories without cloning using git archive - Qiita