Just need clarity, can big players like Foundry USA actually alter transactions on the blockchain? Is this really possible?
Foundry USA or any other mining pool can try to remove a block from the blockchain and they will be able to do so, if they are lucky enough.
Take note that even if they manage to remove a block, that doesn't mean they can steal the fund you sent to someone. Your transaction will be included in another block.
Is there any chance any of mining pools try to do so?
That's very unlikely. They won't do something that can damage their reputation. Also, it's very expensive to perform such an attack and it's more profitable to be a honest miner.
So, If I understood correctly, Foundry USA can remove block that they mined. There might be a case when they mine two blocks in a row and they'll be able to remove both blocks from the blockchain, right? So, technically, 6 confirmation means that there is a very low chance that any miner or pool will mine 6 blocks in a row and that's why 6 confirmation is considered to be a safe choice. Am I right here? If this is the case, then should Bitcoin have beeen made in a way that it will 100% prevent one miner/pool to mine, discover 6 blocks in a row? For better security?
I agree with you that it would be very stupid if any mining pool, especially large one decides to perform such an attack.