If the future is already priced in, how does the average/non-professional person make profit at the stock market?

If everything is known about the market and is priced in, then why does price subsequently change? Why do many professionals traders, money managers, institutions, investment bank trading desks, et al occasionally lose a lot of money (see 2000, 2008, March 2020 and many other time periods)? My two cents is that not only is everything not known but not everyone utilizes the known information the same way.

Take your example that if "good sales for the next quarter are expected (then) the price has already gone up because professional traders react faster." If that was the case, then why do some stocks fall or rise 10, 20, or more points when the earnings announcement is made? If they knew the answer before the EA then there should have been no surprise after the EA.

I'm a bit confused as to what you're after. In one paragraph you're talking about quarterly information and in another, non-professional traders. Investing or trading? Either way, I don't have a simple one size fits all answer for you. You don't have to beat the professionals. All you have to do is consistently much better than money market or CD rates and you're a winner.

The way you compete against the professionals is...you don't compete.

Pick a couple of indexes, pick how you want to distribute your investments using stock or bonds funds that follow those indexes. Put the money in every payday, and then re-balance every year or two.

Don't try to read the market, don't try and time the market, don't try to guess what will be hot next quarter.

Future gains are priced in, but they're priced in at a very high discount rate due to the risk that stocks present.

Consider, say, a bond which will pay exactly $1,000 when it expires 6 months from now. Suppose that some company wants to sell it. Will it ask $1,000 for it? No, it'll ask something like $999, because it would rather have $999 now than $1,000 6 months from now.

We may say that the company is selling the bond at a discount rate of 0.2%, since that's the amount of annual growth you could get by repeatedly making investments that are identical to this bond.

Now imagine a stock. Analysts believe that 1 year from now, the stock will have a 50% chance of being worth $200 and a 50% chance of being worth nothing. Now suppose that some company has some of that stock and wants to sell it. Will it ask $100 for it? No, it will ask something like $90, because it would rather have a certain $90 right now than a 50% chance of $200 in a year.

Now we say that the discount rate is about 10%, because that's the amount of annual growth you could get by repeatedly making investments in a variety of stocks that are identical to this one.

Anyway, individual investors like me are happy to take them up on their offer. I'd much rather have a 50% chance of $200 in a year, than a certain $90 right now.

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$.

This is incorrect assumption. It's generally not 50%. Say the price is 100, market believes that the quarter would be good and company may make x profits, y sales, etc, all of this gets factored in the current price goes up to say 110.

When the results are announced, if it doesn't make the expectations, the price goes down depends on how off could be 105, 100 or even 90 ... However if the company does well than expectations, it goes up may be 115 or 120

It's a continuous evaluation of expectations vs actual.