Soffit:

The soffit is the material that covers the underside of the roof overhang, extending from the exterior wall to the edge of the roofline.

It serves both functional and aesthetic purposes. Functionally, it helps ventilate the attic space by allowing air to flow through vents, preventing moisture buildup and promoting air circulation. Aesthetically, it provides a finished look to the underside of the roof and conceals the rafters.
Soffits can be made from various materials, including wood, vinyl, aluminum, or composite materials.
Fascia:

The fascia is the vertical board that runs along the edge of the roofline, connecting the roof to the outer walls of the house. It serves as a finishing trim, covering the ends of the roof rafters and providing support for the bottom row of roof tiles or shingles. Fascia boards are typically made from wood, PVC, aluminum, or composite materials.

Installation Process:

Preparation: Before installation begins, the old soffit and fascia materials, if present, are removed. Any necessary repairs to the underlying structure are also addressed.
Measurement and Cutting: Accurate measurements are taken to ensure that the new soffit and fascia pieces fit correctly. The materials are then cut to the appropriate size.
Installation of Soffit:
Soffit panels are installed first. These panels may come in individual pieces or long strips, depending on the chosen material.
If ventilation is required, perforated soffit panels or vented strips are used. These allow air to flow into the attic space.
Soffit panels are secured to the underside of the roof overhang using nails, screws, or clips.
Installation of Fascia:
Fascia boards are installed along the edge of the roofline, covering the ends of the rafters.
They are secured to the outer walls of the house using nails or screws.
Additional trim pieces, such as crown molding or drip edge, may be added for a decorative touch and to provide additional protection against water infiltration.
Finishing Touches: Any seams or joints are caulked to provide a weatherproof seal, and any necessary paint or finish is applied to match the overall aesthetic of the home.
Proper installation of soffit and fascia is essential for maintaining the integrity of the roof and protecting the home from water damage, pests, and other elements. It's often recommended to hire experienced professionals for this job to ensure proper installation and long-lasting results.