how do i make a part that if you give damage to them it will spawn parts?

This code can be used to break a part into multiple other parts, for if a part loses health or is destroyed:

function breakPart(part)
	if part ~= nil then --makes sure that the part exists
		if part.ClassName == "BasePart" or part.ClassName == "Part" or part.ClassName == "MeshPart" or part.ClassName == "Union" then --checks if the part is actually a part
			local split = 2 --split the original part into this many smaller parts on each side (for example, it might split it by 2 lengthwise, widthwise, and heightwise)

			local original_size = part.Size
			local original_position = part.Position
			local original_CFrame = part.CFrame

			local original_color = part.Color
			local original_material = part.Material
			part:Destroy() --removes the original part

			local start_x = original_position.X - original_size.X/2
			local start_z = original_position.Z - original_size.Z/2
			local start_y = original_position.Y - original_size.Y/2
			
			local holder_model = Instance.new("Model")
			
			while start_x < original_position.X + original_size.X/2 do
				while start_z < original_position.Z + original_size.X/2 do
					while start_y < original_position.Y + original_size.X/2 do
						local smaller_part = Instance.new("Part")
						smaller_part.Parent = holder_model
						--sets size and position
						smaller_part.Size = Vector3.new(original_size.X/split, original_size.Y/split, original_size.Z/split)
						smaller_part.Position = Vector3.new(start_x, start_y, start_z)
						--sets the color and material
						smaller_part.Color = original_color
						smaller_part.Material = original_material
						start_y += original_size.X/split
					end
					start_y = original_position.Y - original_size.Y/2
					start_z += original_size.Z/split
				end
				start_z = original_position.Z - original_size.Z/2
				start_x += original_size.X/split
			end
			holder_model:PivotTo(original_CFrame)
			holder_model.Parent = workspace
			
			--waits for a set time before destroying the parts
			--remove the next two lines if you don't want it to destroy them
			wait(10)
			holder_model:Destroy()
		end
	end
end

Then, you can implement it in say, weapons, or when a part falls.

An example of how you might implement it in a weapon (Script directly inside a tool):

local debounce = false
local cooldown = 1


function breakPart(part)
	if part ~= nil then --makes sure that the part exists
		if part.ClassName == "BasePart" or part.ClassName == "Part" or part.ClassName == "MeshPart" or part.ClassName == "Union" then --checks if the part is actually a part
			local split = 2 --split the original part into this many smaller parts on each side (for example, it might split it by 2 lengthwise, widthwise, and heightwise)

			local original_size = part.Size
			local original_position = part.Position
			local original_CFrame = part.CFrame

			local original_color = part.Color
			local original_material = part.Material
			part:Destroy() --removes the original part

			local start_x = original_position.X - original_size.X/2
			local start_z = original_position.Z - original_size.Z/2
			local start_y = original_position.Y - original_size.Y/2
			
			local holder_model = Instance.new("Model")
			
			while start_x < original_position.X + original_size.X/2 do
				while start_z < original_position.Z + original_size.X/2 do
					while start_y < original_position.Y + original_size.X/2 do
						local smaller_part = Instance.new("Part")
						smaller_part.Parent = holder_model
						--sets size and position
						smaller_part.Size = Vector3.new(original_size.X/split, original_size.Y/split, original_size.Z/split)
						smaller_part.Position = Vector3.new(start_x, start_y, start_z)
						--sets the color and material
						smaller_part.Color = original_color
						smaller_part.Material = original_material
						start_y += original_size.X/split
					end
					start_y = original_position.Y - original_size.Y/2
					start_z += original_size.Z/split
				end
				start_z = original_position.Z - original_size.Z/2
				start_x += original_size.X/split
			end
			holder_model:PivotTo(original_CFrame)
			holder_model.Parent = workspace
			
			--waits for a set time before destroying the parts
			--remove the next two lines if you don't want it to destroy them
			wait(10)
			holder_model:Destroy()
		end
	end
end


script.Parent.Handle.Touched:Connect(function(part)
	if not debounce then
		debounce = true
		local valid = true
		--check if the part is inside the character
		if part.Parent ~= workspace then
			if part.Parent.Parent == script.Parent.Parent
				valid = false
			end
		end
		if part.Parent == script.Parent.Parent then
			valid = false
		end
		-if the part is not inside the character:
		if valid then
			--Create a coroutine so that the break part function can run in the background
			local taskCoro = coroutine.create(breakPart)
			coroutine.resume(taskCoro, part)

			wait(cooldown)
			debounce = false
		end
	end
end)

You can also implement the break part function in other places that you might want to break a part into multiple parts. All you need to do is do breakPart(part that you want to break). To change how many smaller parts it splits into, you can change the split value on line 4 of the function. You can also change how long the split parts stay in the world at the end of the function.