I'm confused about a lot of terms related to accounting and business?

I am 15 years old and an aspiring accountant. I have been watching a lot of shark tank lately, and I frequently hear a lot of terms related to business. I hear them particularly while watching the show, and also throughout my daily life. Some terms I can think of that confuse me are equity, stock, shareholding, bookkeeping, stock market, royalty, and many others that probably relate to these terms. I have a general idea of what these terms mean, but I don't completely understand. For example, I'm confused on how the stock market works. Is it just each individual company's shares that they sell, or is it something that is kept track of as a whole throughout all companies (if that makes sense)? How does the stock market crash (although, I'm confused on the whole concept of the stock market)? What do shareholders get besides a dividend by investing in stock for a certain company? What do people who have equity in a company get besides a dividend off profits? Is equity another way of saying shares or stocks? I'm just confused on how all these business terms come together.
Added (1). Also, what is a mortgage and reverse-mortgage, as well as a collateral? I know these terms are all related. You don't have to answer all these if it takes too much time, but I would appreciate having many of these terms consolidated a little bit.

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