We recommend installing metal drip edges along the perimeter of the house, the eaves and rakes. This helps prevent water from backing up from the shingles onto the fascia boards, soffit, and siding, which may rot and damage over time from the constant dripping water.
Next, the ice and water barrier is installed at the eaves, valleys, and all protrusions, especially on the north side of the house. The ice and water barrier should extend 2ʹ beyond the interior wall to achieve proper installation practises. Sometimes we install the barrier throughout the roof to prevent ice damming and water backing up, especially in lower-sloped roofs.
Then underlayment should be installed. Many years ago, this was a felt material. These days, we use synthetic waterproof underlayment, preferably from the same single brand manufacture (for warranty purposes).
The next product is the starter shingle. Sometimes roofing contractors use leftover shingles, installing them instead of the proper starter. The problem with that is wind-driven rain can cause roof leaks, whereas proper starter strip shingles have bonding adhesive to hold the shingle above in place and prevent it from lifting from the bottom.