Learn English – Difference between “find” and “search”

Select means to carefully choose; it also means to use a mouse or keystrokes to mark something on a computer screen.

In both the cases, the number of things chosen or marked does not influence which verb is used.

Output is usually used to measure the rate of something. It is often used when talking about energy/power created.

Outcome is the result of an action. The outcome of a fight, for example might be a couple of black eyes.

To try to put these into one context:

This book was the outcome of three years solitude and constant work from the author.

The author has an incredible output, having released 5 books this year and 6 last year.

Also,

The outcome of increasing the machine's printing speed was a much higher output.

Here, by increasing the speed, the engineers have improved the efficiency of their machine.

Hope this helps.

N.B.: It has been pointed out in the comments that output can also be the result of running a process in some computer applications and data processing.