Your post 8 and the reply in 9 was about 'Absolute Maximum Ratings'. These AMR values are NOT to be exceeded without the danger of destroying the device.
Your post 1 was about using a mosfet as a switch, and the AMR are only half the story to help you to select a suitable device. There are many threads here, where the way to cause a transistor to switch and carry a certain current, are discussed. A bipolar transistor needs a certain level of base current to support a level of collector current. These are 'current' devices. The forward gain characteristic is defined as 'beta' which is Ic/Ib, and is dimensionless.
With a fet, the level of drain current is supported by a certain level of gate to source voltage, and the forward characteristic is defined by Id/Vg. This is a 'conductance' and is has the dimensions of mA/volt or milli Siemens(mS) or for big FET's, in Siemens(S).
So, when you use a fet as a switch, it needs to have an adequate Vgs, and not necessarily the maximum the device can withstand. So you need to ask the question; 'what is my drain current and what is the corresponding drive voltage'. When using these things in high speed circuits, you need to consider the rate of change of charge removal from the gate. From your classes in physics, you will remember that current is the rate of change of charge, so this also is a consideration in device selection, because I= C x de/dt. C will be the gate charge.
Data sheets are a hugely important thing to get to understand, and they are full of traps and pitfalls. So you need to keep asking specific questions.