Was this a single data breach ?

No. This is a huge compilation of data from many different sources, primarily gathered over time by infostealer malware infecting individual computers. It was not a single hack on one company.

Are there really 19 billion unique leaked passwords?

No. The 19 billion leaked passwords figure represents the total number of records found in the datasets. This raw count includes many duplicates. However, the number of unique, compromised accounts is still estimated to be in the billions.

Were my Facebook or Google passwords leaked directly from them?

No, there is no evidence that companies like Facebook or Google were directly hacked in the 19 billion leaked passwords news. However, if your computer was infected with malware, your saved password for these services could have been stolen from your web browser and included in this massive collection.

Should I be worried if I use a Mac? Aren't they safer?

Yes, you should still be worried. While Macs have a strong security reputation, they are not immune. Modern infostealer malware is now frequently designed to target macOS as well. Phishing attacks are also platform-agnostic, meaning they can trick anyone on any device. Everyone needs to practice good security hygiene.