What are internal Merge and external Merge in Noam Chomsky’s later theories?

In the Minimalist Program, a framework developed by Noam Chomsky, the operation known as Merge is the fundamental process used to construct complex linguistic structures from simpler elements.1

Merge is categorized into two types:

• External Merge: This is the basic operation that takes two distinct elements (such as two words) and combines them to form a new set.1
• Internal Merge: This is a variant of the operation that takes an element already contained within a structure and merges it into a new position within that same structure. This operation replaced the earlier concept of "Move" (or "Move $\alpha$"), which previously accounted for the displacement of elements within a sentence.1

In Chomsky’s later theories, these two operations, alongside specific parametric settings, form the core of the grammar, effectively replacing the more complex phrase-structure rules and transformational components used in his earlier work.1