Mining race / what happens to "looser blocks/transactions"

how does this mining race work in detail? Ivan shows in the video that mines fill their blocks with eventuelly different transactions (with highest fees) and when they fill the block then they start guessing? who decides when the block is full? or is every miner filling his own block and then starts guessing? so its basically just a race who finishes most blocks and not all are guessing the same nonce for the same block? thats not clear to me. Because ivan says if the winning block is added then other blocks are rejected… but what happens with the transactions in those blocks? what if my transactions ends up beeing all the time in the “looser” block? then it takes much more time? and why actually not simply the most powerfull computer closes all blocks? many questions… hope they are somehow clear :wink: