> You must be at least a very high-level expert in big company like Google or whatever, or you won't know these things so clearly.

No, not at all. This is all basic electronic circuits and basic electrical engineering material. But there are a LOT of basic foundational items required to understand it, and without those foundational items, it is very confusing.

> but didn't explain why computer MUST need RAM from a hardwire level,

It probably did not explain 'why' because this is one of those building block fundamentals the video assumed a watcher possessed.

A computer needs /somewhere/ to store data for later access and use. The conventional name for that is RAM.

> From what I learned the data indicates 1 when that wire got current flow, but since the voltage is still there I think this wire will always has current flow to keep show "1" to our human, until the next circle e.g. a 0 state input come, then no flow anymore because no voltage to send current, am I right?

Your sentence above shows one item of confusion. Voltage and current are not the same (your statement above almost uses them interchangeably). Voltage and current are related (by Ohm's law and resistance for current flows) but they are not the same.

In an ideal system, a voltage can exist without a current flowing (this is one of the reasons that the word "potential" is often used to refer to voltage, a voltage provides the 'potential' for a current to flow, but no current need to flow to have a voltage. (note, I said "Ideal" system at the start, an "ideal system" is one that is perfect, but does not exist in the real world). So keep in mind that a voltage can exist, with a minuscule current flow (small enough to be considered zero, but not really actually being zero current).

In old 5v TTL systems, a "one" is represented by a 5 volt signal on a wire (with very minimal current flow) and a "zero" is represented by a ground level (0 volt) signal on the wire.

So you don't, per. se., have ones or zeros because of current flow, but because of voltage levels (with, in an ideal system, zero current flow).

> You said its the charges which I try to accept but can't. Because later we will talk about RAM, when data is stored in RAM, a 1 bit number like "1" means that wire will keep getting current in order to keep the state.

The fact that you are having a hard time "accepting" that it is charges that are responsible implies that you are missing another fundamental bit of electrical/electronics knowledge. A voltage level is created by having some conductive element contain an excess of electrons (and electrons supply "charge"). An excess number of electrons (relative to some other point in the circuit) creates a "potential" between those two points, and if there is a conductor between the two points, then a current will flow (and the current magnitude is determined by Ohm's law).

> a 1 bit number like "1" means that wire will keep getting current in order to keep the state.

Well, a transistor network will be supplied with a voltage, and the transistor network will be setup such that a "1" being stored holds the transistors in the correct switching arrangement to continue to hold the voltage at the "1" level. But in an idea RAM, there would be zero current in this state, and in a real world RAM, there will be a minuscule current (the leakage current of the transistors).

> input wires only flow a sudden flow of current then fade away, whereas RAM wires will keep let charges flow, but in my eye, all these things build with transistors,

Ah, here might be another source of your confusion. Wires are not transistors, transistors are not wires. Transistors exist, wires exist, and wires interconnect transistors. But they are not the same thing, and both are needed for a RAM. The transistors are connected together using the separate wires to create a particular circuit element (a latch) that can hold itself in two different states.

> I mean, in today's CPU, we has billions of transistors inside, for example, if we want to store the number, 1101, just keep current flow in 1, 2, 4 wires, and done! If we need store another number, 11000101, just choose another 8 wires and keep the states, why not just this?

Because those wires you want to 'use' there are already serving a different purpose. They are the "transportation" network for the data, not the storage.

A useful (albeit stretched) analogy might be to think of a road network. Cars are data bits (and to make it easy, there are only white cars (one bits) and black cars (zero bits). The roads are the wires (transportation to move the cars (bits) from place A to place B), and the parking lots connected to the road network are the RAM (places to store the cars (data bits) until they are needed later).

And assume no designated parking on the roads (wires). So if you attempted to "store" data on the wires, you'd park the cars on the roads. But if you park the cars on the roads, then they block other cars from using those same roads to go from point C to point D.

> why they design like this or that,

In the case of your starting question of "why not just store on the wires" the very simple reason is that the wires are for transportation, not for storage, and using them for storage would create a transportation log-jam (nothing else could use them for transport until their storage function was over).