https://www.berkshirehathaway.com/letters/letters.html
A good place to start is by reading his thoughts on See's Candy as of 2007 (~four decades after he bought it):
https://berkshirehathaway.com/letters/2007ltr.pdf (search for "See's Candy")
Profit only makes sense when considered against the amount of capital required.
The cost of capital is expressed in their balance sheet as expenses or depreciation. Pay back loans, investors, etc are all considered when calculating profit.
For example, a consulting firm may have almost no fixed assets, but let's say its customers are mostly large corps that take 90-120 days to pay invoices. When the firm gets hired for a new project it must cover its expenses for 90-120 days until it gets paid. As a going concern, the firm requires capital equal to 90-120 days of revenues to finance its accounts receivable. If the firm's revenues grow from, say, $100M/month to $120M/month, all else remaining the same, the firm will require an additional $60M to $80M in capital = ($120M/month - $100M/month) / 30 days/month * 90 to 120 days to collect.
So in other words: something like amazon is not a good business for Buffet since they have to finance the inventory for a couple of month at least?
Amazon, in particular, is a large entity incorporating numerous businesses (AWS, Prime, Alexa, Merchant Services, proprietary brands, etc.) that are different from each other. Each should be analyzed on its own, because their dynamics are different. For example, Merchant Services is a platform for third-party sellers. They, not Amazon, are the ones who pay upfront for the inventory. They pay Amazon to store and manage that inventory. When a product in that inventory sells, Amazon gets to collect the money upfront from consumers. The sellers get paid sometime later.
Here's a decent primer on estimating return on invested capital: https://www.morganstanley.com/im/publication/insights/articl...
Of course you’re right that it’s a complex business nowadays.