iOS - Add "objects" to existing app (jailbroken)

Question

How do you add "objects" to an existing app ?

For example, the EasyRefresh for Chrome tweak, enables a new button inside the iOS Chrome app, as do many other tweaks.

How may i add a simple UIButton to, for example, the Twitter app ?

Is there any GitHub projects that might help me to understand how it's done ?

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Image Source: ModMyI

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Thanks.