Ok, let me explain why it is said that only Russia is the source country of India's fifth generation fighter jet technology.
The reason why Russia does not transfer fifth generation aircraft technology to India is because this technology is the most advanced for Russia, and no country is willing to transfer its most advanced technology to other countries.
When the United States successfully developed the F-22, Japan prepared to purchase it, but the US legislation refused to export the F-22 and instead exported the slightly inferior F-35. This is because in the 1970s, the United States sold Iran its most advanced F-14, and later regretted it, even destroying all retired F-14s to prevent Iranians from buying F-14 parts on the black market. Now the United States only allows its allies to purchase F-35 instead of selling technology.
When Pakistan purchased the J-20 from China, China politely refused and instead sold the J-31/35 with lower performance. Similarly, China did not transfer technology to Pakistan.
Nowadays, Russia, like the United States, hopes that India will purchase finished products or assemble them in India. India hopes to acquire all the technology, which is obviously impossible. Only when the Russians have developed better technology can they possibly transfer the existing technology to India, which takes time.
For the United States, even though the F-16 and F-15 are no longer as advanced, the radar, electronic systems, engines, and other technologies used in these aircraft are not sold to the outside world. For example, the engine used in the KF-21 from South Korea is the F-414, not the more advanced F110 series, and South Korea only uses it without transferring production technology. If India applies to the United States for more advanced engine technology, they will only say two words, it's impossible, and pay more money.