Using Alphabet's 2019 10-K Report below - I know the company's major sources of cash and major uses of cash in each of the three sections (operating activities, investing activities, and financing activities) in the statement of cash flows. Can you help me understand why each is like this and what it means?

Operating activities :-

• Major source = Net income
• Major uses = Account receivable

Investing activities :-

• Major source = Maturities and sale of marketable securities
• Major uses = Purchase of marketable securities

Financing activities :-

• Major source = Proceeds from issuance of debt, net of cost
• Major uses = Repurchase of capital stock

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https://www.sec.gov/Archives/edgar/data/1652044/000165204420000008/goog10-k2019.htm