I assume these are two different questions.
I'm on Android and tapping on a message and holding your finger there for a few seconds will open a pop up that allows you to copy the text in a message.
As for taking a screenshot, it depends on the exact phone you're using, although holding the power button on most phones opens some commands (power off, restart, airplane mode...) and one of the commands allows you to take a screenshot. Another way to do it on some devices is to hold down the power and volume up/down buttons simultaneously. I suggest you Google it with the name of your phone. For example "Samsung Galaxy S9 how to take screenshot".