There will be no difference for SEO to choose one or the other approach for your URLs.
However, I think it will be better to choose this schema because it seems logical to me:
mydomain.com/companies (main category explore section)
Indeed, the category page refers to a list of companies and a detail page for Nike is just a presentation of one company belonging to all listed.
Moreover, with this approach, you can implement your website architecture without the URL problem of choosing a different one for each category page.