How do mining rewards get verified? I understand that a miner who adds a new block can pay the reward to their address per the coinbase transaction, but who verifies that claim? Say, for example, I add a block, but instead of claiming 6.25 bitcoins, I claim 50 bitcoins. Who is going to verify that my claim is wrong? Thanks.
I add a block, but instead of claiming 6.25 bitcoins, I claim 50 bitcoins. Who is going to verify that my claim is wrong?

Every full node will validate every block and enforce that it adheres to the consensus rules. Such a block would be rejected, because it is invalid.
For example, the network rejected an invalid block in July 2019 when Antpool mined a block that claimed transaction fees but forgot to include the corresponding transactions.