It seems a common canon, that when you get to know information about the future of something at the stock market, that the price at the stock market already has this information calculated in. For example if it is expected that the next quarter a company will sell less and therefore be worth less, the price has already gone down, and if good sales for the next quarter are expected the price has already gone up because professional traders react faster (because of having more time to follow the news of the companies and better news sources, better analyzing tools, ...).
Also for example for bonds the price per coupon seems to be original price + remaining dividends - likelihood/risk of non-payment for example because of bankruptcy.
Simplified there is 50% chance expected that the value after the quarter announcement is $90 and a 50% chance the value after the announcement is $110, so the value before is $100. So you buy it at $100 and hope you are lucky and it will be worth $110. If you are unlucky you lost $10 per share. On average it will be $100.
So to me it seems that for a non-professional it is just a gamble with an average 0 payout (before fees). Still many people seem to use it, for example for increasing money or in the USA for pension because of the missing pension system in the USA.
Are those just speculators or what am I missing in my conclusions/where am I wrong/how does the non-professional person beat the predictions of the professionals?
(I hope I didn't hurt anyone by the use of some terms because English is not my native language and I am not that familiar in this area)
I am asking for investing/holding a position for longer terms like at least multiple months/years, not day-trading.
Even if a non-professional doesn't compete with professionals/doesn't want beat them they influence it so they can only buy it at a price that already includes expected future gains if I understand it correctly. But still people seem to make money with it. This is what I don't get.
Please keep in mind that I am a beginner and have a beginner flaw in my thinking or not familiar with all effects/consequences in the market.