They have to seal the chamber when combustion takes place, but also provide as little restriction as possible to the incoming air/fuel mix.
The basic requirements for a valve are to be as light as possible, especially for high-revving engines and to that end can be formed from titanium.
The design of the back of the valve head, where the stem meets it, needs to match the shape of the port. For example, a relatively shallow port into the chamber will want a shallow valve head, to allow air to flow over the back of it and straight into the chamber. But a steeper port may want a thicker, more tulip-shaped valve head to guide the flow around the valve edges.
The valve seats are what the valves themselves actually seal against and are usually hardened steel rings, machined into cylinder head material. They get a fair pounding as the valve closes, especially at high engine rpm.
The actual seat is a 45” angle but flow is improved by detailed machining into and out of the seat. Even better flow through the valve/seat area can usually be achieved by either a three-angle or a radiussed seat with a flat sealing face cut into it.