No, they are not the same.
Traditionally, developments used to be programmed by “directly modifying” the code base so that the system would do what the user wanted. But the implication that this change in the base could have in the face of a version upgrade was rarely taken into account.
An extension, on the other hand, is also programmed to meet a user’s expectation and need; but, unlike a development, the focus is on never modifying the code base, but on “extending/extending” it, in such a way that the standard application will never be affected in its own updates by incompatibility with the extensions.
In short, extensions have come to replace developments.