You’re not meant to be able to modify the Roblox hotbar. Occasionally there are hacky ways to do it, but those usually end up getting patched.
Everything you can turn on can be turned off using StarterGui:SetCoreGuiEnabled. You can see the things this can control here (ex: the player list, the chat window, the emotes menu, the captures button, etc).
You can also add to the default roblox hotbar using ForeverHD’s Topbar Plus (e.g. add additional menus that match the default topbar).
If by the hotbar you mean the tool bar, you can disable it using the SetCoreGuiEnabled then create your own (there are some resources on the forums of custom tool bars people have made).