Android is built on top of the Linux kernel, thus companies are required to provide the source code, upon request, of any Linux kernel binaries they distribute on devices they sell. Most major OEMs do fulfill this commitment by making the source code publicly available, but the uploaded source releases often lack critical components and/or full commit history. Nevertheless, the availability of the kernel source code is often useful for aftermarket developers, and Realme is doing a pretty good job nowadays on this front. The company has now released the kernel sources for 4 devices: the Realme 6, Realme 6 Pro, Realme C3, and the Realme 5i.
Realme 6 XDA Forums ||| Realme 6 Pro XDA Forums
The Realme 6 series features a 64MP main camera, 90Hz display, and support for 30W fast charging. The 'Pro' model comes with a slightly larger display and a more powerful processor (Qualcomm Snapdragon 720G instead of MediaTek Helio G90T), while both phones run Android 10 with Realme UI out of the box. However, AOSP lovers should eventually have a vast selection of custom ROMs to choose from, made possible by kernel source drops from Realme.
Realme 6 Kernel Sources || Realme 6 Pro Kernel Sources
Similar to the above two phones, the Realme C3 also runs Realme UI based on Android 10. The global variant of the Realme C3 is, hardware-wise, quite different from the Indian model, but the kernel source code should be unified between the two of them.
Realme C3 XDA Forums || Realme C3 Kernel Sources
The Realme 5i is the odd one out here, partly because of its software. Despite being announced in January 2020, the phone shipped with Android 9 Pie-based ColorOS 6.1. According to the company, Realme UI should land on the Realme 5i in May, alongside the Realme 5/5s. These three phones are essentially identical, but Realme has opted for a separate kernel tree for the 5i.
Realme tends to release device-specific bootloader unlock tools (aka In-Depth Test) with the kernel source, but there is no such announcement thread in their forums as of now.