When you add skills, in my experience, it adds them to your account, not each echo product individually. So you only need to add them once through one device.
The thinking behind these little guys is a bit different than building a computer and they are a completely different kind of complex. Don't think of them as a computer. think of them as children. You have to teach them what todo. The problem with that is, you have to know their language (Which is very literal in some instances, and very loose in others) So much is already native to the echo devices, you really don't have to teach them much, but you do have to learn how they "think" a little. Adjusting *how* you say things will come along slowly, as they require some specificity in language (whichever you speak and use). They also learn your habits, and can sometimes start "skills" without as much prompt as they took the first several times.
There's no*one way* to use the Echos, as everyone who uses them has different uses for them, from information (news and weather etc...) to entertainment (music, audio books, etc...) and smart home control such as turning on lights and setting them up to routines, and turning on the TV or coffee pot or such... too much to cover in one simple post for sure.