Do they know there are protests?

We do know. The number of mainlanders that know about the protests is probably larger than HK's entire population.

If you can read Chinese, go to weibo or tieba or tianya or hupu, you can see posts about the protests. Surely they may get removed after a while but I can assure you that one doesn't gets arrested just for posting the news and discuss about it a bit. The number of off-line discussions cannot be estimated.

Proof (a screenshot from NGA): https://imgur.com/a/FbofCOK

Please remember that HK is physically connected to mainland China and 83.2 million people crossed the border in 2015[0]. Hell, even in 1984 some people still know the government's dirty secrets, it would be extremely arrogant to assume we know nothing.

[0] https://en.wikipedia.org/wiki/Lo_Wu_Control_Point

That must be how the explain the protests, lol

The same thing that people in areas far away from protests typically think about protests. Unless you are personally involved, or know someone who is personally invested, what you read in the news has a big influence on your opinion.

Domestically, OWS may have been a big deal on both of the left coasts, but in the middle of the country, people thought it was a bunch of lazy entitled liberal children asking for free handouts.

Strange. In Europe a lot of people supported OWS.