No power to dash, no headlights, no car start, after installing a new battery?

It sounds like you have popped the main 40 amp fuse in the fuse panel under the hood. Can I ask if the battery got connected backwards by chance? Here is a guide to help you check the fuse for power and the location of the fuse in the images below.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Also please check to see if one the power leads from the battery didn't get reconnected besides the positive battery cable. Check out the images (below). Let us know how it goes.

I think I did connect it backwards because I have a RHD and not LHD and most images online are for an LHD. I got a new main fuse (120amp) because it was blown. I now have power to the whole car, but it still won't start.

My main fuse was blown. I replaced it but as I'm putting back in fuse bolts on side there are no threads, so the bolt(10mm) doesn't stay in. The other (8mm) side has threads and the bolt stays. I am getting power to the car, but it won't start or crank. (I wasn't getting any power before)

Is the security light flashing? Also, can you check the remainder of the fuses? This guide can help you, please check the fuses with the key on:

https://www.2carpros.com/articles/resolving-reverse-battery-connection-in-cars-a-step-by-step-repair-guide

and

https://www.2carpros.com/articles/how-to-check-a-car-fuse

They have maxi fuses with threaded holes, but can you upload an image so I can see?