Rewrite and Redirect base path to folder

I have a website like

https://example.com

running a React SPA so the /<*> is replaying to index.html which is working great.
I also connected

https://othersite.com

which is also going to /index.html

however I would like to be able to route

https://othersite.com -> https://othersite.com/a/1

However trying to do so then causes a chain of /a/1/a/1/a/1. My goal is that the address bar shows the /a/1 site, but everything is still relayed through /index.html

Start by reviewing AWS Amplify Hosting rewrite and redirect behavior for custom domains and React SPA fallback rules. Reproduce the two-domain setup, then verify that the second domain displays /a/1 in the address bar without repeatedly appending the path while requests are still served through /index.html.

Written by the indexing model from the issue text.