Is there a way to create your own method/function etc?

You can create your own metatables emulating this behavior.

Create a ModuleScript, in it write:

local Module = {}

function Module.new(age)
    -- Age is an example parameter
    -- This new function will return a Module object
    -- And this function works as a constructor for this Module object
    local self = setmetatable({}, Module)

    self._age = age

    return self
end

function Module:GetAge()
    return self._age
end

return Module

:GetAge() method returns the current value of the _age property of the initialized Module object. As you can see, it uses a colon, which is a shorthand for .GetAge(self) (note the period instead of a colon). It's basically syntactic sugar.

Remember to accept the answer if is it helpful for you! I'm glad to help :)