Does Apple use a third party technical support company called WebSupport24-7.com?

You gave crooks control of your Mac. There's no telling what they did while on your system.

Right now - and I do mean RIGHT NOW - you need to change the passwords on critical sites, such as banking.

But without knowing for certain they didn't install a key logger, turning on Screen Sharing or some other method to track what you do, you must first erase and restore the computer to a point before the intrusion. It does no good to try and change passwords if they can see what you're doing.

No it’s not legit. Apple doesn’t need third party tech support companies. Hopefully someone in the Community can advise you on what to do next.

The notice you received was just an ad, and you should have ignored it. It didn’t “lock up” your Safari.

See: Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Have a great day!

I allowed them to control my computer to fix security warning.

—If you do online financial transactions, you need to notify your banks and credit card companies of a potential breach and ask them to issue you new account numbers. Granting access to the scammers mean they likely have a number of your once-secure financial credentials.

— From now on, NEVER assume a bank or credit card statement is 100% correct. Take the time to carefully examine every charge on all your bank and credit card statements for possible fraudulent charges. You may first see small (less than US$5) charges from fast-food places or a parking meter on the other side of the country. Then you get hit with a big charge from a retailer with whom you've never done business.

I speak from experience. We found that my wife's AMEX card number was pilfered from a bankrupt retailer. Its online sales site had been improperly shut down when the company went out of business, leaving thousands of card numbers easily accessible for mining by skilled hackers. That's not an internet rumor; the info came directly from AMEX specific to our incident.

These financial news articles discuss the decade-old "mystery of the unknown charges " scam:

https://www.foxbusiness.com/personal-finance/credit-card-statements-scam-charge

https://www.chron.com/business/moneytips/article/Check-Your-Statements-For-This-Long-Running-Scam-13888458.php