Q: I just got a warning supposedly from Apple saying that a site I visited had infected my computer. Is this legit?

No, any such warning will be a scam. Do not respond to those messages.

Phony "tech support" / "ransomware" popups and web pages

Close Safari. If necessary force Quite.

Then reopen Safari with the Shift key depressed to prevent opening previously opened windows

Beware of Browser Pop-Up Tech Support, Phishing Scams

Phishing & Other Suspicious Emails

Fixes for Adware and Pop-ups:

1. Malwarebytes
2. DetectX 2.11
3. Remove adware that displays pop-up ads and graphics on your Mac
4. Stop pop-up ads and adware in Safari - Apple Support