I was planning to use a subdomain provided form infinity free to test out my SEO content writing and do trial and error experiments. I would buy and add a .com domain if I enjoy doing these but want to stick to the free domain provided by infinity free for now. Will using this free domain hurt me SEO?
No
you can always tell google which is a “real” version of your website
and through 301 redirects all the previous search results google will switch to a new address