How do I add notes that always show on my invoices or estimates?

Use the default notes field. Go to Business settings → Invoices (and Estimates, separately) and fill in the Notes field. Whatever you put there is pre-filled into the notes section of every new invoice or estimate you create — payment terms, thanks, legal disclaimer, anything you want at the bottom of the document.

The notes are a default, not a lock, so you can still edit or remove them on any individual document before sending. Changing the default later only affects new documents. Existing ones keep whatever was on them at the time.

If you also want bank account or payment instructions in there, see Adding bank account / payment instructions to invoices. For the full set of invoice customisation options, see Customising invoices and estimates.