I've read the threads about canonical root issues
And here:

and I think I understand the reasons why.
Recently I've observed websites that are redirecting with a 301 from example.com/ -> example.com/index.htm
They have all of there internal links pointing to the /index.htm page on the site so for me this is the same as redirecting from /index.htm to / so why is Google indexing the root / URI and not the full path /index.htm .
Would this cause an issue for Google and slow the indexing of the site, I just don't understand if the 301 is pointing to the index.htm why Google needs to index the example.com/ as the homepage of the site.
Is there any real difference as long as there's only one root or does Google's Algorithm insist that I use example.com/ as my root URI now. Any advice greatly appreciated.