If you need to take someone or something out of one photo, and place them in another photo, it takes some finesse to make it look right. The size has to be right, the angles have to be plausible, and the colour and direction of the light can make or break the job. All of those things are fodder for another post on another day.
Right now, I want to talk about the cut-and-paste job itself.
Adobe give us plenty of ways to select part of a photo - magic wand, pen tool, lasso tool, marquee tool, quick mask, quick selection tool, color range, etc, etc. With each new version of Photoshop, the options become more numerous and powerful. Over time, you'll master some or all of these methods.
Here's the thing - if you want to put part of one photo into another photo, it's easy to assume that you need to cut that part out, then copy it across.
If you do it that way, you need to be 110% certain that your selection is perfect before you cut and paste. And frankly, it's impossible to be certain about that.
No, the best method is to copy the whole image across (or a generous relevant portion of it, anyway). Then use your favoured selection/extraction method to create a mask on that layer, to simply hide the unwanted background. This way, you've got infinite control over your selection, and if you discover that you messed up and gave somebody a three-fingered hand, or something, you can easily modify the mask to fix the problem.