In SN1, bromide leaves before any nucleophile arrives. A carbocation forms.
SN2 is different. The nucleophile hits the back side as bromide leaves, all at once.
Now bulk it up. Three methyl groups wall off the back, so SN2 stalls.
Those methyls also stabilize the cation, and polar protic water molecules cradle it.
So SN1 wins with tertiary carbons, weak nucleophiles, and polar protic solvents.