I also only know next to nothing
However, as I understand it the share is supposed to reflect the value of the underlying company; it's assets, it's income (existing contracts, revenues), its debts. Owning a share means you effectively own a part of the company (there are different share types and you don't really own a part of the company afaik). If the company were wound up (i.e. all assets/stock/products etc were sold off) you'd get a percentage of that sale... after any outstanding debts were paid off.
This is the point where someone like RM can likely point out if we're wrong/false assumptions/part way there but missunderstanding bits and bobs
So in my understanding that's why when they go "oh crap we're making less money than we forecast for this quarter/year" the stocks generally fall in price. That's because the underlying value of the company has decreased as the believed value of said company likely includes expected returns/profit over a certain time period as against the companies expected outgoings (rents, wages, pensions, costs). Hence how Tesla is so valuable despite in past years the volume of cars being sold is very low (company valuation worked out at like millions of dollars per car sold, whereas Toyota is about 20k per car sold). The fluctuations may depend if it's temporary (a ship blocking the Suez) or more permanent decline/profitability (shift to smartphones which Nokia didn't lead, and therefore got left behind... same with Blackberry) meaning their ability to make/sell and therefore profit dried up, or innovations/developments that in the future may pay off big time (shift to electric, battery advances, or even just R&D that can be licensed) mean it could be worth more with expected growth/profitability, so the price rises as it's more attractive to gamble on that future.
Why the share price moves so much - buying and selling I always thought. It's worth what someone is willing to pay for it, and things like wars, sanctions, price rises (manufacturing), wage rises, market trends, seasons, disasters (natural or other) affecting manufacturing, logistics for shipping), riots, can all impact upon a companies ability to operate efficiently, make a profit, or remain profitable in the face of competition and their costs. Hence why when everything seemed to be going south with Covid the markets tanked for days on end. If you can't trade, or no-one is producing required components for manufacturing your product, then you can't make money... and if you've wages to pay (or planes to pay for as per airlines) without any income, then... once the company runs out of money it's bankrupt and those shares are worthless. Then Govts finally rode to the rescue to help ensure companies wouldn't go bust via loans and wage help schemes, shares started rising because the spectre of bankruptcy has been postponed.
On a day to day basis the companies have costs to cover, and that requires cash. That cash is via profits, or loans/debt from banks and other lenders (or issuing shares to raise funds from investors) - which can influence company value and therefore likely profitability (servicing your debt) and therefore share price as attractiveness to investors. Hence why when some companies are in trouble they may issue more shares, so investors who believe the company can be turned around into profit, will hopefully buy and supply the cash required. Otherwise they have to sell off assets, and if that doesn't address why it's losing money, it'll not really cause much. Is also reason why people at the top of companies get replaced when "performance is poor" (translation, share price isn't great) as they hope a new person with new ideas/ways to cut costs/grow business, can drive company forward and improve the company once more.
I think markets have mechanisms to suspend trading in shares if the drop is too big in a given day/timeframe, to prevent exploitation (i.e. you own 10% and try to sell that 10%, there's unlikely enough willing buyers at present price to take it off your hands, especially if bad news hit about the company, so the price keeps dropping until someone is willing to buy it - but other investors may also sell believing something is seriously wrong, so prices keep going down... and then it causes a company to collapse maybe? that's a bit i don't get, or alternatively the initial seller then buys at the new super low price to exploit the situation and profit). Likewise I always assumed a rise reflects how much you'd have to pay to persuade an existing shareholder to part with their share.
Share issues are a means of a company to raise funds. They offer X shares and investors pay for those shares (possibly just below the market price in order to make it attractive to investors to actually buy them), and the reason it shouldn't affect the price of the shares is because that transfer of money from the investors is now held as liquid cash by the company. It's worth the X it was prior to the issue and each share represents %age W of company. The share issue means company worth X + cash raised of issue, and the %age W should remain equal on a per share basis. So a flotation involves raising money for the company to invest in it's growth, and the people buying the shares factor in the believed value of the company + the cash the share issue will raise + faith in the ability to invest said cash in a way to grow the company. That's why some companies do well on flotation (as all matches) and some don't (as the latter one they wonder where the income will come from, thinking Facebook's initial flotation - the concern was huge audience, how to monetise because if they can't then...).
So that's my understanding, and I've written it out not because I think it's absolutely correct, but because if it's not and someone has better understanding then hopefully they'll share and I'll learn more too