How would I make a charging blast like this?

I won't answer the whole questions, but i sure will point you in the right direction to creating a script like this.

First you will need a couple of Particle Emitters to be used as the effects shown in the example you gave us, then you will need a script to Detect either a KeyDown or MouseClick so it can begin to charge the blast using a loop constantly increasing it's size while the variable is true. If your still confused, here's an example.

local UIS = game:GetService("UserInputService")
debounce = false -- used so the script can only be ran ONCE at a time
canCharge = false
chargeVal = 0 -- gonna be using this to help increase the size and let's you now what number its up to

local function onCharged(Input, gameProcessedEvent)
local KeyCode = Input.KeyCode
if not gameProcessedEvent then -- checks if the player ISN'T typing

    if KeyCode == Enum.KeyCode.E and debounce == false and canCharge == false then -- change E to the key you want to be pressed down
        debounce = true -- prevents the script from being ran over and over
        canCharge = true -- gonna be used for a while loop
        local growingPart = Instance.new("Part", workspace) -- creates a part parented to the workspace
        while canCharge do -- while loop that only runs if canCharge = true
            wait(0.1) -- change '0.1' to how long you want to wait until the value is increased
            chargeVal = chargeVal + 1 -- adds 1 to the chargeVal while canCharge = true
            print(chargeVal) -- prints the chargeVal's value each time it is being increased, since it's in the while loop

            -- you can make it so it can increase the size using +Vector3.new(X, Y, Z). An example is given here :
                growingPart.Size = growingPart.Size +Vector3.new(.1, .1, .1)

            if chargeVal >= 50 then -- conditional statement to check if chargeVal is greater than or equal to '50'
                canCharge = false -- ends the while loop since it was used to check if canCharge = true
                print("Fully Charged!") -- prints Fully Charged
                wait(3) -- time until the script can be ran again
                chargeVal = 0 -- starts the chargeVal back to 0 so it can be fully looped until '50'
                debounce = false -- allows the script to be ran again after 3 seconds
                canCharge =  false -- allows the script to be ran again after 3 seconds
            end
        end
    end
end
end

UIS.InputBegan:Connect(onCharged)

If you have any other questions, please feel free to ask!

--oSy

This script uses Instance.new and it takes time for developers to make these things "the things going around it, how would you do it?": Simply use a particle emitter and put the id of the particle you want and then change some other settings to make it look like in the video