You can type notes at the bottom of the account timeline where it says "Type an internal note".

If you'd like a dedicated field for Contacts or Opportunities (or Accounts), you can create this. 

Just head to "Settings" > "Customize fields", select the right entity, and click on the large orange "+" button to add a custom field. Call it "Notes" and set the type to "Large text". It can fit a book. 🤓

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