Why do original legs have orange dots, but mirrored ones do not? I tried to follow a tutorial about Bones but I had to delete them and recreate them from my memory but now I have a problem. Please look at my photo! I've got 4 legs mirrored via "modifier" and 4 bone structures which I duplicated on the other side. I parented everything for a bit more overview.

Now I'm confused why the original legs on the right have those 4 fat orange dots, I think I need them on the other side, too! What did I do wrong?
The orange dots are the object origins of the selected objects. It looks like you have created multiple armature objects, one for each leg.

You only want one armature object for the spider, then add multiple bones to the armature while in edit mode. This allows you to move the entire spider as one while moving individual parts in pose mode as needed.

You can select multiple armature objects and merge them into one by pressing &#9096; CtrlJ