Moving repo A into another repo B without erasing repo B's existing code

[lflo5727]

Probably a dumb question, but I've been going over the docs and can't seem to figure it out.
I'm trying to move a repo (we'll call repoA) into an existing monorepo. My goal is to use git-filter-repo to move the repo and its history. What I think should work according to the docs is:

git-filter-repo --to-subdirectory-filter apps/my-repo-a --tag-rename '':'my-repo-a-' --target /../path/to/monorepo

Now this moves repoA to the monorepo/apps/my-repo-a folder like I want with all the history in tact. However it completely nukes the contents of the monorepo, making repoA the only thing existing within it.

What am I missing? I understood that as --target using --partial that it would maintain both histories and code.
Also, more of a side question, it only seems to affect the default git branch. Is there a way to make these changes on a specific branch so that I could keep my new changes in a separate branch and open a PR into the main branch with it?

Any help would be appreciated.

[lflo5727]

I was able to resolve this using both git-filter-repo and git merge.
I used git-filter-repo to restructure the file tree in my-repo-a to match the structure it should belong under in the monorepo.

Then in my monorepo I added the local path to my-repo-a as a remote

git remote add my-repo-a ../my-repo-a

After that I ran git fetch my-repo-a --tags and then merged.

git merge --allow-unrelated-histories my-repo-a/main

And that did exactly what I needed. Got all the files where they belonged, nothing was overidden and the full history was intact.