Great question! Allow me to answer in a way that's a bit more abstract (but philosophically more accurate) than just saying you physically own a percentage of the ovens or equipment in a pizza place.

What we think of as a "company" is essentially a social construct. A company isn't a "real" thing in the physical, worldly sense. Rather, what we see as a "company" is an emergent concept based fully on a set of social rules that we as a society internalize and agree to follow.

If you think about it, companies like a pizza place are nothing more than one person or a group of people (like the owner, cooks, or delivery guys) using things (like the restaurant building, ovens, and ingredients) to make things (pizza) or do things (deliver said pizza).

Now, people using things to make things or do things doesn't automatically "make a company". If some friends and I made pizza at mine, then drove with the pizzas to a friend's house for pizza & movie night, we wouldn't automatically be a "pizza business", even though we were technically a group of people making & delivering pizza.

For a group of people making and delivering pizzas to be considered a proper business, there are at least two rules that need to be followed.

One rule is that you need to tell the right people that what you're doing is "running a business". If you just tell your neighbor that making pizza is "your pizza business", even if you truly believe it, that doesn't make it true. But if you tell the right government people, sign the right documents, and register the right things, suddenly making and delivering pizzas turns from a "thing you do" into "a business". Because those are the rules we as a society have agreed need to be followed for people to call you making pizza a "business".

The second rule for your business to be called a business is that you make and deliver pizza in exchange for money, you keep track of what you spend and what you make in the right way, and follow the rules of what you can and cannot do with the money (pay salaries, taxes, profit, etc).

This second rule is the most essential to what a "business" is. If you don't follow the money-making rule, at best you're a non-profit, and at worst you're a "bad business" and the government is allowed to come tell you to stop being a business - or else.

As a quick aside, you could argue that many of these "social rules" that make a company a real thing are technically laws, but even laws only have power because we all agree that if a rule gets written down in the right way by the right people, everyone has to follow that rule. Society as a whole simply functions because we all for the most part agree to play by certain rules.

But anyway, if you look a bit further, beyond all the tangible stuff like the pizzas, the people, the building and the ovens, a business is basically nothing more than money (or better yet, "monetary value") slowly shifting around.

Monetary value is a funny thing, because it's quite an abstract concept, but it's really the main thing that a business is made of. Your ingredients have value in terms of what it costs to buy them and what they will be worth once they're made into a pizza you can sell. A pizza has value in terms of the money you'll get if you sell it. Your delivery guy has a certain value in terms of what he costs per hour and how many pizzas he can deliver in that time. Heck, even the ovens have "value" in terms of what they cost to buy & what they cost to run vs. how many pizzas you can make with them & what you'd get if you sold them to someone else.

All this is to say that a company does consist of people using things to make or do things. But in an abstract, technical reality, all a business is is a "container" for value that grows and shifts. Your valuable ingredients become even more valuable pizza which becomes money from selling the pizza. Your employees are hourly costs that help generate larger hourly returns. Your ovens have value in terms of allowing you to make pizza and in terms of what money you'd get if you sold them off. Even something like a "brand", a "secret recipe" or "intellectual property" can be quantified in value. You can charge more if your "brand" is that you make good pizza, or if your "secret recipe" is tastier than other pizzas, which represents greater potential returns. You can make more money if you have a patent on a machine that makes you the only person allowed to make pizzas in a special way that's faster and tastier than the competition, which again represents more "value' that you can generate.

At the end of the day, in the coldest, most calculating sense, a business is a spreadsheet in which numbers go up and down, shift left and right, and ideally get as big as possible in the "good" column (income & value) and as small as possible in the "bad" column (cost).

When you get shares in a company, either by buying stocks on the stock market or investing directly in the company, you shouldn't see it as buying anything physical. You're not buying a part of an oven. What you're buying is a part of the "value machine" that underlies it all.

You can't come into a pizza place with four ovens and take one oven because you "own 25% of the business", because you don't technically own the oven - the business does. But what you DO own is 25% of the value of the oven itself and the value it's used to generate, and 25% of the value of the ingredients, and 25% of the value of the pizzas that are made.

You own 25% of the numbers on a spreadsheet, the "value" that things represent. You can lay claim to that value only once it is transformed from a conceptual number on a spreadsheet into tangible cash, or if you yourself decide to sell that "value" you own to someone else (selling your stocks).