you'll need to read up on taxonomies, and how to classify data. There is scant resources on wikipedia surprisingly, but if you are looking at say ecommerce have a look at the amazon browse by dept. Here is a sql list of it on github

You can see just for shopping/retail Amazon has over 8,000 categories.

Then there's the now defunct DMOZ which is not a bad start even though its out of date.

You can probably scrape some directory websites with a bot and aggregate the data. That's how I got started.

I initially searched on Google with 100 results per page then saved the source HTML and wrote scripts to strip and save the data to a database. You can find common strings in the source and urls of CMS's for example shopify uses /collections/ and woocommerce uses /product-category/ so you can use "inurl:/collections/" on google to find shopify websites.

For paid lists there's builtwith (expensive) and whatcms (very reasonably priced in comparison).

Its quite labour intensive as it requires a human to determine what category a website falls under, so the more you can automate the better. I'm currently looking to apply a browse by department for my website using amazon's categories and modifying it to suit. I have over 10,000 uncategorised websites in my app so am working on categorising them now.

There's a lot of business directory projects on github so have a browse there.

In the country I live there are a few paid business directories, I don't know how they get all their data but it is mostly not publicly available so they sell the data and its quite expensive. I think I read somewhere that the UK govt has a free API for accessing company records, so check for that as well depending on the country you're in.