https://blog.devgenius.io/how-to-merge-two-repositories-on-git-b0ed5e3b4448
Step 1: Clone one of the repositories (say repo1).
git clone <https/ssh-link-for-repo1>
Step 2: Create another remote in this clone which points to repo2-our second repository.
git remote add <your-custom-remote-name> <https/ssh-for-repo2>
Now you will have the following remotes
origin <repo-1-link> (fetch)
origin <repo-1-link> (push)
your-custom-remote-name <repo-2-link> (fetch)
your-custom-remote-name <repo-2-link> (push)
Run : git remote -v to check the remotes you have
Step 3: Fetch content from repo2 into this remote
git fetch <your-custom-remote-name>
Step 4: From remote to a local branch
Note that the content of second repo is in the remote still, which we fetched but can’t yet access. So we copy it onto a new local branch.
git checkout -b <new-branch-name> <your-custom-remote-name>/master
Check for files if you have to, see if you’ve correctly pulled in the right data.
Step 5: Merge with master.
Notice that the last command had you checkout of the master branch and into a new-branch. So, first checkout from that to master with git checkout master
then run: git merge <new-branch-name>
Now it may return this error
Fatal: refusing to merge unrelated histories
Since we are trying to merge two separate repositories, git checks for co-relation between merge and commits histories and blocks the merge process.
You can surpass this with the following flag with the last command
git merge <new-branch-name> --allow-unrelated-histories