I wonder how the following scenario is handled in bitcoin/crypto: Let's say evilminer has 10% mining power of a coin and he secretly mines blocks while the real chain is growing from blockheight 900 to 1000. He starts with a faked block 995 and stops at block 1005. At Blockheight 1000(real chain) he publishes his fake chain.A node would now request/get the next block. And it would get now block number 1001 from evilminer.
What would this node do with it? I think to know if the chain made by evilminer to proof wrong or true it would have to download block 1000 to 95 to finally see that block 95 was fake.
Is this correct or is there another way a node would handle it?
The situation you have stated would never arise.
If you want to make your chain the longest, then you need to have more than 50 percent of the mining power, this way sometime in the future you will eventually catch up with the main chain and surpass it. The speed of catching up will be dependant on how much more than 50 percent you have. This attack works because this way you have beaten any other potential miners in the rate of producing of new blocks, so you are always producing blocks faster than anybody else on the network, hence ensuring that your chain will become longest.
So your evilminer, given that he has 10 percent of mining power, would never be able to advance the length of his chain beyond the main chain. The only other possible way is to use a nonce of lower difficulty to speed up block production, but such a block would be rejected by any other node in the network deeming it as invalid.
Hence, in short the only way evilminer can make other nodes accept his blocks is to make his chain the longest, and that can only be done if he has more than 50 percent mining power of the network.
Edit 1 : Consider the computation power of every participant of the network and assume that when you sum these up, you get 100. Now if you have 10 percent of this total, it means your likelihood of finding a new block is higher than others who have lesser computation power than you. However, this does not guarantee that you will definitely find the nonce first.
Finding the hash that satisfies the difficulty depends on probability. You brute force your way through a large set of numbers until your hash satisfies the difficulty. It maybe possible that someone having mere 0.5 percent of total computation power finds the nonce first. However, the faster you can crunch these numbers, the higher the chance that you will find the nonce first.
So if somehow you have more than 50 percent of total computation power, that means you are processing these hashes faster than anybody else, and hence this increases the likelihood that you find the nonce first most of the times.
Also, this process of finding the nonce, is repeated for every block. So the work has to be done all over again for every block.