there are so many linux distros, most of them are free,
they need software engineers to develop the OS, but it was given free,
so,
how do they make money to pay for their workers' salary?

It probably depends on the distro, but for example Ubuntu relies on people to volunteer and work for free, and are funded by Canonical, who provide technical support and bespoke systems for companies (or, I guess, individuals). I just checked their website and it says the fee "goes towards the continued development of Ubuntu", so this might mean that they do pay the volunteers some money, or it might mean that it goes towards things like advertising or producing CDs to distribute the OS. But I had always been under the impression that the developers worked on a voluntary and unpaid basis...

nobody will work without money, i think so.
even non-profit, but u need money to fund your expenses,
so, from where come the money?

support contracts, mostly. (look at RedHat Linux series or SuSE Linux series to name two, they offer (for companies relying on support, a support deal, with a yearly subscription, the software itself is gratis, but the subscription is not)
Also they get money (e.g. redhat) by going into partnerships with oem's etc.. and get some money from there.
Or simply financial funding deals, or stock trading (nasdaq e.g)

Many open source developers do it in their free time. I'm an open source developer but I have a job that has nothing to do with open source. Take Martin Nordholts for example. He's a developer at Gimp in his free time but according to his Resume he is currently a Software Engineer at Sony Ericsson Mobile Communications.

Many projects are funded by donations. Donations can go anywhere from paying for server resources, renewing a domain, or simply go into the developers' pockets (which it should since they work on it).

There are other ways as well. I've gotten a support request from a company in one of my open source projects and they said they would pay anyone for the support. I answered, they did, and I was paid well. Hopefully this sheds some light but IMO open source comes from the heart.

Much of the Linux kernel revenue comes from the server market and generous donations from companies like Novell and Red Hat.

think of it like a music player. say guitarist.
there are a good bunch of videos on youtube of people recording themselves playing.
they win no money , and they do it in their spare time as a hobby or to get better.

THEY=developers

haha, maybe chinese people are too realistic, no money no do?
anyway, i admire those who contributed.