1. The most basic condition for an anchor chain to work is to hook something on the seabed. If the anchor chain is not long enough, the anchor will not work. If the seabed is flat, or if the anchor is not fixed, or if the hook is not too firm, it can be fine if the waves are calm. However, if the waves are too large and the anchor cannot hold onto something, it will cause the anchor to lose its function, which is called "anchoring". During the anchoring period, it is very dangerous for the ship to anchor because the main engine is usually stopped when the ship is anchored. If the ship sails immediately, it will take time, and it is very dangerous for the unpowered ship to drift around. So, that's why the concept of anchorage came into being. Anchorage implies that the seabed is relatively rough, and it also needs to be able to shelter from wind.
2. The weight of the anchor chain is negligible for a ship, and the frictional force will not have much effect. Generally speaking, when a ship is anchored, the anchor chains are straight, which can basically ignore the effect of friction. If you are by the seaside, you will find many small fishing boats with thick anchor chains.
3. From the ship, the anchor chain appears straight, but there is a section underwater that is close to parallel to the seabed (in fact, it is attached to the seabed). The anchor provides grip, which is then transmitted to the ship through the anchor chain to resist the impact of external loads such as ocean currents and wind waves on positioning. The reason why there is a section that is attached to the seabed is to consider the influence of force. Think about it, with an anchor, pulling the anchor chain straight makes it easy for the anchor to move. And there is a section that is attached to the seabed, which can provide a margin within a certain range.
