In response to GhalibKabir's fine comments, of December 9, 2020, on nuclear weapons proliferation to-from China, I add further comments below:
Finding US Overt sources is quite easy. Finding non-US sources is more difficult.
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SOURCE 1
Mainly https://www.nti.org/learn/countries/china/nuclear/
“China was embroiled in nuclear proliferation scandals throughout the late 1980's and early 1990's, particularly with respect to its sale of RING MAGNETS to Pakistan in 1995. [38]
[Wikipedia - "A subsidiary of the China National Nuclear Corporation contributed in Pakistan's efforts to expand its uranium enrichment capabilities by providing 5,000 custom made ring magnets, which are a key component of the bearings that facilitate the high-speed rotation of centrifuges. China has also provided technical and material support in the completion of the Chashma Nuclear Power Complex and plutonium reprocessing facility, which was built in the mid-1990s.[50]" ]
[Missile Threat In November 1992, China sold 34 M-11 [aka. DF-11] missiles to Pakistan. Satellite images showed missile canisters were delivered at Sargodha air base near Lahore.13 The Chinese government initially denied these allegations, accusing U.S. intelligence agencies of fabricating evidence of the transfer. The United States reimposed limited sanctions in response.14 After Beijing provided a more explicit commitment to adhere to the MTCR guidelines and to stop exporting missiles, the Clinton Administration lifted the sanctions in 1994.15 Although China pledged not to transfer missiles to Pakistan, it reportedly continued to assist Pakistan in developing its own indigenous missile capability.16 In 1992, China attempted to sell to Iran 500 DF-11 missiles. However, the deal was terminated due to U.S. pressure.17 A reported 30-50 DF-11 missiles may have been sold to Iran in 1995, although this remains unconfirmed.18 ]
China provided Pakistan with a nuclear bomb DESIGN (used in China's October 1966 nuclear test).
[That test was CHIC 4, October 27, 1966, Warhead 548, 12kt test yield]
These designs were later passed to Libya by the A.Q. Khan network, and discovered by IAEA inspectors in 2004 after then [Libyan] President...Qadhafi [who very foolishly - look what subsequently happened to him and his country!] renounced his nuclear weapons program and allowed inspectors to examine related facilities [in Libya]. The plans contained portions of Chinese text with explicit instructions for the manufacture of an implosion device. [39]”
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SOURCE 2
Mainly https://www.atomicheritage.org/history/chinese-nuclear-program
Concentrates (at length worth a SubMatts' article in itself) on help to China from a US traitor, perhaps semi-official 1940-60s far left Curie cluster? French helpers and (no doubt permitted by the Soviets) the brainy but malleable, Klaus Fuchs (using his stolen from UK/US knowledge and expertise)
Source 2 goes onto help FROM China
"...during the 1990s. On one visit, he was told that the CHIC-4 (see above) the bomb used in China’s fourth nuclear test—was designed simply enough so that “anybody could build [it]” (Reed and Stillman 231).
Chinese scientists passed the CHIC-4 bomb technology to Pakistan, and allegedly conducted a nuclear test for Pakistan at Lop Nur on May 26, 1990. Additionally, China sold intermediate range ballistic missiles (IRBMs) albeit without nuclear warheads to Saudi Arabia, sold missile components to Iraq [can't find details], and trained Libyan nuclear experts in Beijing [maybe not much evidence?]. China has also tolerated the North Korean nuclear weapons program; after Pyongyang’s first test in 2006, China’s ambassador to the UN affirmed that “China does not approve of inspecting cargo to and from the D.P.R.K.” (328)."
[China - North Korea relations is an ongoing story. With China thinking even a nuclear armed North Korean Buffer Zone (between China and US-SK forces) is better than no buffer zone at all.]