Often I see EV given for hands in a particular spot.

Perhaps a hand may be an open because it's EV is +0.01bb/hand so you open it, and a hand slightly below this may be -0.02bb/hand so you fold it for example.

Are these EV numbers found from assuming that your opponent is a perfect solver opponent?

If so, then what does the presence of bad players at the table mean for the EV numbers in practice? Does it give all of your hands an EV boost, so now a hand which should be +0.01bb/hand is actually +0.06bb/hand making that hand an even better open than it was before, and a hand that should be -0.02bb/hand is now +0.03bb/hand and so should be opened rather than folded?

Hmm good question.

I think, if we take a hand like 54s; it is played for board coverage. And it is a hand which is ment to be played aggresively, to benefit from fold equity since we dont have any SDV.
But if you are playing against a fish, having board coverage really doesnt matter anymore, and playing more high cards (for exemple openning more Axo) seems better to me since we will have more SDV with A high. therefore, against bad players, my guess would be that we would make more money with high cards, therefore having more EV with these hands.

And a hand like 45s could lose some EV against a fish since he is less likely to fold to multiple streets of aggression, therefore making this hand a lot harder to be profitable.

Having bad players doesn't automatically boost the EV of all hands, because they may put you in lines very often that reduce the EV of some hands.
But yes, all of the EVs will be different

Presumably it is the more speculative hands that lose EV, and the better high equity hands gain EV?

It depends on how the guy plays, if he 3bets a lot, then all of the threshold hands lose EV and we should open tighter.
If the guy is very aggro postflop, hands that can't get to showdown often versus triple barrels will lose EV, and hands that can, will gain EV.
If the guy folds a lot then all hands gain massive amounts of EV...
There are almost countless possibilities