Union Government
(1) Subject to the provisions of this Constitution, the executive power of the Union shall extend - (a) to the matters with respect to which Parliament has power to make laws; and (b) to the exercise of such rights, authority and jurisdiction as are exercisable by the Government of India by virtue of any treaty or agreement: Provided that the executive power referred to in sub-clause (a) shall not, save as expressly provided, extend in any State to matters on which the State Legislature also has power to make laws. (2) Until otherwise provided by Parliament, a State may continue to exercise executive power or functions it could exercise immediately before the Constitution's commencement.
The Union government's executive (implementation) authority generally extends as far as Parliament's law-making authority, plus whatever rights and obligations arise from international treaties India has entered into - while generally staying out of matters where states also have law-making power, to preserve the federal balance.
Because foreign treaties are covered here, when India signs an international agreement (like a trade treaty), the Union government - not individual state governments - has the executive authority to implement and enforce India's obligations under that treaty.