Is the merkle root in the header dinamic ?

Miners combine transactions and include the mercle root in the header and start trying to find the proof of work when changing the nonce. Are the transactions included in the block candidate dinamic or they are already preselect when the miner is hashing ? From my understanding they are already selected and added to the template and the miner only changes the nonce with every hash. But if they are already preselect how when i broadcast a transaction with high fee its immediately included in the block candidate and not in the next round when miners construct new template with transactions from the mempool?