Redirecting to the home page is not a good idea. It is bad for SEO and it is bad for users.
Google calls redirects to the home page "soft 404" errors and treats them just like regular 404 errors. Google does not pass link juice from the old site to the new site when you redirect to the home page.
Redirecting to the home page of the new site is bad for users because they can't find what they were looking for and they don't get a meaningful error message. Bookmarks and inbound links break in a non-obvious way.
Instead of redirecting to the home page, you want to individually redirect each page to the equivalent page on the new site.
https://newsite.example/ (home page redirect)
https://newsite.example/blue-widgets.html (Redirect where path changes)
When a page has been removed with no equivalent page it is best to show a
410 Gone status with a custom error message like
oldsite.example/content/blue-widgets was removed. Please visit newsite.example which has our new product line.
In any case, it is always more than just a DNS setting to enable redirects. DNS can only point a domain to a web server. It is always up to the web server if URLs should get redirects, error pages, or content. The web server can base that decision on the requested domain name and path.
You need to use Route 53 to point
olddomain.example to your IIS server. Then you need to configure IIS to properly handle each URL from the old domain.