There are only limited quests that involve solo instances, if that is what you are asking. Outside group dungeons and trials pretty much all can be done solo but other players will be doing them too. As MMOs go, ESO is very solo-friendly indeed, but if you want to do all the quests without others around you then MMOs are clearly not as suited to you as single-player CRPGs would be.
I've no idea frankly where your question is coming from, but hopefully between @Runschei and me we've covered all possible angles.