The problem with the way this is taught in schools is that it is normally taught as a single grammar structure that you can't separate. First, let's look at the parts

除了
“除” basically means "to get rid of". examples are worlds like 排除，消除
Therefore “除了” basically means "other than" or "except" (probably more literally means "getting rid of ____")

以外
literally means something like "outside of". remember “以前” means before and “以后” means after.

This combinate of words, effectively says "except for.... other than that", which may be a little repetitious, but that's the way it works.

This is used in almost the exact same way as English's "except for" and "other than".
Usually these are either used to show:

an exception to a rule (i.e. something that doesn't fit with others) (this uses 都)
Except for him, none of us has money
Except for him, all of us has money.

or to show variety by explaining that many more things are present. (this can optionally use 也 or 还)
Other than apples, we also have oranges and bananas
Other than her, 5 more people are coming.

So... using these definitions lets translate a few sentences!
除了他以外我们都没有钱。 Other than him, we all don't have money.
除了他以外我们都有钱。   Other than him, we all have money.

除了苹果以外，我们（也/还）有橙子和香蕉。 Other than apples, we (also) have oranges and bananas.
除了她以外，（还/也）有5个其他的人要来。   Other than her, 5 other people are (also) coming.

So, in sentences like the first two, you should almost always use “都” because you're expressing that everyone/everything is the same with one exception. In the 3rd and 4th sentences “还” and “也” are often optional but add extra emphasis, so are usually more natural.

If you have any questions, feel free to ask!