Several developers, miners, and researchers reported that Bitcoin mining developer name Marathon Digital mined an invalid block 809478.
means the marathon digital got 6.25BTC for mining an invalid block?
do the invalid block which got mined is placed in a real bitcoin blockchain or any other blockchain? or not added to any blockchain?
and if the invalid block was mined, so of course there were wrong transaction(or transaction from 0 balance wallet) or double spending was happened?
Blocks that don't get included in the main chain are not rewarded on Bitcoin like they are on some altcoin chains. The invalid block would have caused a temporary chain split but the chain which included the block from Marathon would have quickly died off because it is following incompatible rules and nobody would have continued mining on that chain. After the block is rejected from the valid chain everything continues as if it never existed and its transactions remain unconfirmed in the mempool until they're included in a valid block.