How and how long do these companies that always make pre-ICOs millions of Dollars advertise their ICO?
you can see the roadmap of his project, usually there is a schedule from the developer about the start time until after the pre ico or ico. can you see in ann thread or whitepaper.
Do they really advertise for two weeks on social media and with signatures and then they raise so much money?
time does not have to be two weeks, because the campaign schedule has been set by the developer and bounty manager.
yes, every project must have a bounty program, bounty is a good marketing strategy for an ico. but not just social media campaigns and signatures, there are many other campaigns, such as content (video and blog), translators and much more
How does it work? Or do they find those big investors?
with the campaign program and also other ads (google adsense) then the project will be seen by investors and if investors are interested then they can join in ico
Is there anything that I do not catch?
a lot of things that you do not understand, try to read a lot in this thread, if you do not understand you can see and read on the local board (your home country)