Hi init w and welcome!
Yes, the sidebar tabs are regular tiddlers that are tagged $:/tags/SideBar
One way to access all tiddlers tagged something specific is to use the tag-pill macro by, in a tiddler, typing in <<tag "$:/tags/SideBar">>. In it you can click on the concerned tiddlers and once you have the tiddlers you can remove the above mentioned tag from them so they don't appear there. And vice versa, if you want a new tiddler there, just create one and tag it such.
Because these are tiddlers that build up the system they are super easy to restore; just delete(!) the tiddler you've manipulated and the original will be reinstated.
<:-)