How do I make something that happens in every server? (Globally)

DataStores

Probably the worst way you could do this would be using DataStores, mainly because of their request liimit. It is something like NumPlayers*10+60. The only way you would be able to use this feature is using a loop constantly checking a value, in intervals of around 30 seconds to a minute.

E.G;

function setGuis(message) -- function for setting the message
	for _,v in pairs (game.Players:GetPlayers()) do -- iterates through players
		local text = v.PlayerGui.MessageGui.Message -- gets the gui to change
		if message then -- if there is a message
			text.Visible = true -- shows gui
			text.Text = message -- sets text
		else
			text.Visible = false -- hides gui
			text.Text = "" -- removes text
		end
	end
end

local ds = game:GetService("DataStoreService"):GetDataStore("GlobalMessage") -- gets DataStore
local key = "message" -- gets a key for the DataStore

function setGlobalMessage(msg) -- sets the global message
	ds:SetAsync(key,msg)-- sets the message
end

while wait(60) do -- repeats indefinately
	local msg = ds:GetAsync(key) -- gets the message
	setGuis(msg) -- updates guis
end

P.S:

Maybe it isn't as bad as I made it out. It can just be flawed at times, and the request limit isn't so bad when you look at it.

Trello/Other Service

I don't believe there is a request limit for HTTP, so you can use it trello and other services in the same manner I made above.

I haven't tried to use this before, so I can't really help you, but a google search can help you with that. SH also has a useful blog post on that: https://scriptinghelpers.org/blog/logging-errors-with-trello

Hope I helped!

~TDP