I don't understand how is merge-mining work, where can find some detail?

Merged mining allows a miner to mine more than one coin at the same time. Merged mining only affect mining of the coin. It does not affect the amount of coins produced, the block times, the block rewards, the price of the coin, the usage of the coin, the community, or the development of the coin. Only miners would have to care about merged mining. Regular users would not need to know or care that their dogecoins were produced at the same time as litecoins.

First, the concept of mining is basically you keep hashing the block you are working on with a random nonce, until you get a hash that's smaller than a target value. The higher the mining difficulty, the lower the target is, so the longer you have to keep hashing to find a hash that small. Litecoin's difficulty is higher than that of Dogecoin, so for the same hashrate, you will find a dogecoin block (if mining dogecoins) sooner than a litecoin block (if mining litecoins) on average.

What merged mining does is it lets the miner mine on both blockchain at the same time. If the miner finds a hash lower than the Dogecoin target, then it can submit that block to the Dogecoin network as a valid block. And if it finds a hash lower than the Litecoin target, then it can submit that block to both networks and get rewarded for both blocks. This is because a hash lower than the Litecoin target is also lower than the Dogecoin target. So in essence, you get twice the reward for the same amount of work. The problem is if everyone switched to merge mining, then the difficulty will adjust accordingly. So you still get both LTC and DOGE, but you get about half the amount of each. So it all evens out in the end. The first people that switch to merged mining will get an initial bump in revenue, which will taper off when more and more people switch over.