Understanding the Search Space for Valid Block Creation

Hey all, I’m diving into solo mining for Bitcoin and got stuck on a couple of points. 1) What exactly are the degrees of freedom with finding a valid block? Like, what’s the real search space we’re dealing with? 2) Is there any assurance that within this search space a valid block is gonna be found eventually? I know miners use the "getblocktemplate" RPC to get a block template and that the "bits" field helps calculate the target hash that the block needs to be below. But I need a clearer picture! Anyone can explain this better?