You didn't want the survival stuff inside the car because it could become missiles, so everything was packed in the trunk.

Then the car slid off the icy road and over the bank, and now you can't open the trunk.

I've heard you can open the back door and remove the back seat, giving you access to the trunk, but how is that done? Can you do it by hand (or do you need the tools that are also in the trunk )?

(This isn't a discussion on where to keep your stuff, BTW.)

Most car seats unbolt from the floor and the back cushion lifts to remove it. For them you need a wrench (yup, the wrench locked in the trunk).
Some seats just clip in place and don't even need tools to get out. If you really had to I suppose you could cut the cushions out of your way with a knife and wire cutters.
Whether you can get through whatever is behind it is another question to answer. Most of the cars I have had would at least let you reach an arm in.
The car I have now (Pontiac GranPrix) has a middle seat in the back that folds down and makes an opening into the trunk. No tools needed. It was an option to let people put longer items inside the trunk.

I try to keep a minimum of things inside the passenger compartment because my Volvo S60 sedan's fold down rear seats are locked and can be unlocked only from the trunk. This includes the middle seat pass thru. I think this is for valet security since I can set the car to not open the trunk when using the valet ket.

So I keep a good sized FAK under the driver's seat, a multitool in the glove compartment and a Maglite in the seat pocket. But all the stuff in trunk is out of reach if I can't get to the trunk release.

On the bright side, if I ever get carjacked and thrown in the trunk I'll have everything I need to effect my rescue...

The only trunk we have is on the wife's car and it has a "ski" port between the rear seats to slide your ski or snowboard through... I can fit enough of myself through to reach anything inside and pull it through. Found that out by accidentally closing the trunk on the keys after changing the tire!

My Escort has a fold-down rear seat to extend the cargo area, that can be released from the passenger compartment. Between that and the 14" crowbar under the front seat, I should be able to get in there.

(Yes, the crowbar is secured against "aerial antics". )

My SUV has no trunk! My supplits are in a tough plastic storage(s) box secured by cargo straps in the rear.

My Toyota Camry has fold-down rear seat (split in two independent halves). My son who sits on the rear seat routinely folds the other half-seat down to bring something from the trunk while I am driving.