Seen vids of people playing this online and do not know what level of choice to play they have chosen
as in Easy hard normal etc, but when the main character gets killed in a fight he just gets up and
carries on from that in game point.

I assume they are playing at lower playing levels, would that
be so ?

I'm on the second tier difficulty... what I presume is the normal level;
I have to reload if my guy dies.so I think you are correct, assuming he wasn't just knocked down.

This happened to me on Death March one time, when I stole from a chest and a guard in the house, who I hadn't seen, started attacking me. I thought it was bugged because Geralt was just laying on the floor dead for a while, then when he got up I assumed it was something that happened with certain encounters. I reloaded anyway, just to be safe.

I think it has something to do with certain quest missions.

Normally you die and have to redo the mission.
On a couple of occasions, Geralt has just got up again.

I've not had that happen yet; I'm curious to see it.