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