what's the practical way to get ahrefs for free?

Ahrefs Webmaster Tools is the legit free version. Verify your site (connecting Search Console is the quickest way) and you get Site Explorer and Site Audit for your own domains, backlinks included. It won't give you full competitor data. Their free checkers (backlink checker, keyword generator) show a slice of it, which is sometimes enough. I'd avoid the shared group-buy accounts people sell. They get cut off constantly and you're handing your login details to strangers.