Screenshots is nowadays a basic feature that is included in Stock firmwares(ROM). Most big players like Samsung, Sony, Motorola, etc. include a stock method to click screenshots on your android device. These methods are usually have to be found out by accident as they aren’t usually listed anywhere in manuals or documentations.
I’ve made a list of Different Manufacturers and the methods to click screenshots in them. These methods are generally applicable to all phones of the manufacturers however some exceptions always remain.