Find and replace
Searching the document or every open tab, and a replace that keeps the capitals of what it finds.
Ctrl+F⌘F opens the search bar at the top of the window. Type and the matches are highlighted as you go, with a count of how many there are and which one you are on. EnterEnter or the arrow buttons move between them; on Windows and Linux F3 does too. EscEsc closes the bar.

The search ignores capitals and needs at least two characters. It does not use regular expressions or whole-word matching: it is for finding your way around a text, not for editing it by pattern.
Replace
The small arrow at the left of the bar unfolds the replace field. Replace changes the current match and moves to the next; All changes every match in the document.
The replacement keeps the case of what it finds: searching harbour and replacing it with port turns Harbour into Port and HARBOUR into PORT, unless the replacement itself has capitals, in which case it is used as typed.
In all tabs
Tick In all tabs and the search runs through every open document, with the count of matches per tab. Replace then works one match at a time, and All is disabled: a change across every document is one to make deliberately, match by match.
More in Writing
The Markdown shortcuts that turn into headings, lists and emphasis while you type, and the ones you can turn off.
The small toolbar that appears over selected text, and the reminder of shortcuts that shows when you hold Ctrl or Cmd.
Six levels of heading, the panel that lists them, and the keys that move through it without touching the mouse.
Bulleted, numbered and checklist items, nesting them with Tab, and turning one kind of list into another.
Links to web pages, to other documents on your disk and to headings in the same document, and how to follow them.
Four ways to put a picture in a document, the difference between embedding and linking one, and what the editor does not do with images.
The slash menu that inserts blocks, the toolbar that edits tables, and the rules that keep them compatible with Markdown.
Notes attached to a passage, in three colours, which travel with the document and never print.
Curly quotes, dashes, ellipses and arrows written for you as you type, the quotation marks of your language, and how to turn each one off.
The spell checker and its languages on each system, adding words to the dictionary, and the language of the app itself.
Right-to-left writing for Arabic, Hebrew, Persian and other scripts, per document or for every new document.
What happens to formatting when text comes in from elsewhere or goes out, and the two ways to copy a document as code.
Calmly Writer