“When Grant showed up at the battlefield he turned the tide...”
You obviously are not familiar with Shiloh. “Grant developed a reputation during the war for being more concerned with his own plans than with those of the enemy. His encampment at Pittsburg Landing displayed his most consequential lack of such concern—his army was spread out in bivouac style, with many of his men surrounding a small, log meetinghouse named Shiloh Church, passing the time waiting for Buell’s army with drills for his many raw troops without establishing entrenchments or other significant defensive measures. Major crossings into the encampment were guarded and patrols frequently dispatched.
In his memoirs, Grant justified his lack of entrenchments by recounting that he did not consider them necessary, believing “drill and discipline were worth more to our men than fortifications.” Grant wrote that he “regarded the campaign we were engaged in as an offensive one and had no idea that the enemy would leave strong intrenchments to take the initiative...
Grant wanted to avoid provoking any major battles until the linkup with Buell’s Army of the Ohio was complete. Thus the Union army had sent out no scouts or regular patrols and did not have any vedettes in place for early warning, concerned that scouts and patrols might provoke a major battle before the Army of the Ohio finished crossing the river. Grant telegraphed a message to Halleck on the night of April 5, “I have scarcely the faintest idea of an attack (general one) being made upon us, but will be prepared should such a thing take place.”” - Wiki
Grant was already there and in command. But he didn’t expect the South to attack...so he did NOTHING to prepare.
He was there the first day. His orders indicate he didn’t know enough about what was going on to give useful orders. And that evening, 15000 additional men arrived...and the commanding general on the Confederate side was dead.
You might try reading a few books about Shiloh before blaming Sherman for Grant’s decisions. And before claiming Grant performed well.
Grant was not there when the battle started. He was 20 miles upriver. Sherman was the one who ordered no entrenchments and advised Grant there was no evidence of confederates nearby. Maybe you should actually study the battle.