I usually respond to email in Thunderbird, and there I know how to intersperse a response or comment within the message.
How does one do that in Gmail (in the browser)?
Some of this will depend on your personal settings. Assuming yours are close to mine, here's one way to do what you want to do.
Click the button that looks like an ellipsis at the bottom of your reply pane.
(If you hover your mouse it will tell you that it will "Show trimmed content")
This will expand the quoted text for you. Quoted text is indicated by a line on the left margin
Place your cursor in the spot where you want to break up the quote and hit Enter twice. This will form a break in the quoted text. Add whatever part of your reply you need to here.
If, for some reason, part of the reply becomes "unquoted", you can re-apply the quote format by selecting the text you want to change and using the "Quote" tool under "Formatting options" (or Ctrl-Shift-9).