what's the practical way to choose an seo company?

Ask them what they'd do in the first 90 days for your site specifically. A decent agency will have looked at it and will talk about things they found. A bad one talks about packages and "number of backlinks per month". Red flags: guaranteed rankings, no access to your own Search Console and Analytics, vague about how they build links, long lock-in contracts. Ask for two clients you can actually speak to, and make sure you own every account and every piece of content if you part ways.