Selection Mode

If you want to copy some measures, the Shift/click will put the box around all that is selectively included.

To copy, then press Ctrl and C.
Then click on the first instance of where you want them to appear and press Ctrl and V.

They should appear there.

Or, perhaps I don't understand? :)

FWIW, there isn't anything called "selection mode". But you can indeed make selections while in "normal mode" (the mode you are if you are not in note input or edit mode), and if you see the blue box, you have successfully made the selection. And you can indeed do copy and paste and lots of other things, just as you can in other programs. Eg, Ctrl+C to copy, Ctrl+V to paste, as described above.

BTW, "Ctrl" would not normally be involved in making a range selection (the kind with a blue box that includes everything in the range) - only a list selection (individually selected elements that may or may not be anywhere near each other, no blue box). So if you want to use copy and paste, don't use Ctrl in making your selection.
Click one element, Shift+click another to select everything between those elements - just as in most other programs.

Click below for my Quick Answer on how to make selections:

Make Selections

https://masteringmusescore.com/quick-answers/make-selections

Click below for my Quick Answer on how to copy, cut, and paste:

Copy, Cut, and Paste

https://masteringmusescore.com/quick-answers/copy-cut-and-paste

What it sound like you may be looking for is the repeat selection button which is R. It will duplicate what you have selected starting on the next beat. It works for whole measures, or chords. As with all copy and paste operations, it must be a continuous selection and what is repeated is left on the clipboard if you want to paste it somewhere else.

You wrote:
*I thought selection mode allowed you to repeat measures with repetitive patterns or ostinati,etc. *

Here's an example (follow the written steps):
Repeating_patterns.mscz
Be aware that you may need to use the selection filter to exclude items you don't want repeated.
(Not the case in this example.)

Regards.