How it works
Remove line breaks replaces each break, and any spaces around it, with a single space, so the lines run on as one paragraph. Join into one line does the same but also drops blank lines and the spaces at each end of a line. Then press Collapse spaces if double spaces are left inside the text.
Keeping the paragraphs
Both buttons join paragraphs too. To join the lines inside each paragraph but keep the blank lines between them, open Find and replace, turn on Pattern, find (?<!\n)\n(?!\n) and replace it with a single space. That pattern finds a line break with no other line break next to it.
Questions
What about words split with a hyphen at the end of a line?
They stay split, as exam- ple, because the page cannot tell a broken word from a real hyphen. Fix those by hand, or, when you know the text has no real hyphen followed by a space, find - and replace it with nothing.
Can it put the line breaks back?
Undo brings back the text as it was. It cannot rewrap text at a set width; your editor or email app does that.
Is my text sent anywhere?
No. Every change happens in this page on your device, and nothing is saved once you close it.