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 "Long text". It can fit a book. 🤓