It's been many years now and my memory of how it works is hazy, but isn't the unlocking of stuff in survival done by playing the campaign and unlocking stuff in the tech tree there?

And of course you can respec the tech tree at any time so you can pretty much unlock everything without too much bother if you get far enough.

i mean i'm asking because i complete level 10 with something unlocked but not the other things. but then i complete level 10 again with other things unlocked. the things i have before reverted to being locked.

I think it could be the case that it uses whatever loadout you have researched at your highest achieved campaign level.

I guess playing on Easy Campaign doesn't unlock anything then. I always thought that Endless mode was just broken. All these years later, still have never played that mode.

(The normal campaign levels are quite difficult)

Easy doesn't, no. The main campaign is not as hard as it looks ... provided you remember to spend spend spend! Hoarding is what makes it harder - it thinks you're doing well so it chucks more and more monsters at you.