If this is what you are talking about, it is your power steering reservoir and yes it takes ATF fluid
not power steering fluid
As for the stiffening plate on the bottom you need to get a spanner in above it to hold the nut and you will need to get the car up on ramps as you need to lie down underneath the car and contort your arms in ways you probably haven't before (Mechanics have lifts which would make it that much easier)........
You don't need much traction on the nut you just need to get a spanner onto it, I recommend a flare nut spanner like this
(Can't remember the size though I think it might be 16mm)
once it's on you can just make sure it stays on the nut and crank the bolt from below, let the spanner spin with it and eventually it'll butt up against a part of the chassis or undercarriage and will hold the nut still from there. Same deal to get it back on except put your hand in between the spanner and the chassis so that it doesn't jam in place.
It is heavy so don't be under it when you undo the last bolt, if you have it on ramps use your jack to hold it up and then it should stay in place even with all the bolts removed, then you can let down your jack, it's almost impossible to get back into place by yourself without a jack so may as well get the jack ready