first one statement is that. they people are not playing only with a single website. they might have some other kind of primary business/job, when you see no adverts or no fee on any website/forum.
@How they earn through forum/site?
as above statement, it's not sure shot that one is getting great revenue from a site. specially when there is no adverts or no fee on it. these sites are mostly (but not every) used to promote there primary business to the public.
@From where/How they pay huge amount of charges for their bandwidth and all that?
very simple. no need to worry. if the forum/site is very up-to-date. then be sure that the owner of that site is paying for it, but dont be sure that the owner is getting revenue from the same site. they have some other source. like above said.
@How the companies came to know that on this website we have to give advertisment ?
some sites take this thing in there own hands to get advertisers directly. in this case they get more revenue from advertising if implement good strategies. you call it marketing, exactly. if this is the case then companies know very well that where they are going to adevertise.
but on the web in today's market the above case rate is lower but the bigger case we have third middle man that is marketing for us.
very well known is Google with it's advertising programs AdWord(for advertisers) and AdSense(for publishers)
A company owns Google Adword account to advertise there products on the web.
and an other company(forum/site) owns Google AdSense account to publish adverts on them.
both(advertiser and publisher) are given a fully functional reporting system to see alternatively (advertiser)where their ads are being published and (publisher)how much they get revenue from it.
@Is this forum or website thing can be implement as EARNING SOURCE for owners (forums/sites)
I think you can answer this question your self now.
this purely depends on the owner's mind and their current situation that they even need to earn from it or not.
on the other hand there are also some other factors to evaluate that the earning should start from a site or not or when to start earning from a website.
I hope I answered all of your questions.