Q. If there are 100 miners around the world and 100,000 transactions in the mempool, do all the 100 miners pick up the same 4000 txns in the block sorted by fee? (I read online that a block has approx 4000 txns). I would imagine that all 100 miners would pick up different transactions?
Once a transaction is included in a block, that's no longer in the mempool and the next block will include new transactions.
So, a miner includes 4000 transactions into a block and they are removed from the mempool.
The next block will be mined (by the same miner or a different miner) and will include 4000 other transactions.
Q. If the USA government controls these miners and tells them to not pick up transactions from the mempool that belong to the addresses blacklisted by the US government, can't a Russian miner create their own block with the transactions from the mempool ?
Miners can include any transaction they want.
I don't think it's going to happen though.
I also don't think that will ever happen. But let's assume it will happen.
Let's say the miners following the rules own 70% of the total hash rate. Miners who own the other 30% of the total hash rate are located in sanctioned countries and don't join the 70%.
What do you expect to happen? Will they stop mining? Or there will be a hard fork and they will create a separate a chain?