where does "my pending balance" stay until I receive my payment in mining pools? what is the procedure? where/how pool keeps the pending balance?
You can think of the pending balance as merely a number on a spreadsheet. First, some details:
Pools can choose different methods to allocate the reward amongst the miners. The most common method of allocating miner rewards is to allocate it to miners only once a block is found and is mature (about 2 hours old).
Continuing that example, every time a block is found by the pool, the reward is recorded on the blockchain as an output owned by the mining pool's address. The mining pool owns those funds until it decides to pay miners.
The mining pool software run by the pool is, of course, tracking how many shares are submitted by each miner. It is therefore easy for the pool to calculate the number of shares submitted by each miner as a percentage of the total shares contributed that resulted in the block being found.
Once the two hours is up and the block is mature, the mining pool will retain its stated percentage of the block reward as its fee, and then will credit all the miners' pending balance with their respective percentage of the remainder. In this sense, the pending balance is merely a number on a spreadsheet.
To reiterate, the pool owns the Monero until it has been disbursed to miners. Miners trust that the pool operator is honest, that the pool operator is allocating shares correctly/fairly, and that the pool operator will make payouts according to the stated schedule or threshold. Miners, therefore, appreciate it when pool operators use open source pool software.
So, over time, miners' accounts on the "spreadsheet" are credited with their respective portion of the mining rewards the pool earned. Once the pool's payout threshold is reached for each miner, the pool will issue payments to that miner. Generally, a miner can expect a small delay after they have reached their threshold, since it is more economical for the pool operator to wait and send payouts to multiple miners at once, rather than one at a time.