I would highly recommend learning following things:
Win32 API
Networking (Communicate over HTTP/s, DNS, ICMP)
Encryption (basic use of Aes, Xor, Rc4, etc.)
Injection Techniques
Learn how to use Debuggers.
Read the source code of already existing open source C2s like Metasploits Meterpreter, Empire Framework, SharpC2, Shadow.
These projects contain so much info and code on how to:
make malware modular using reflective loaders/code injection, communicate with the C2, and more.
Here are all of my personal malware development resources i have collected:
https://github.com/rootkit-io/awesome-malware-development
https://github.com/rootkit-io/malware-and-exploitdev-resources
https://www.youtube.com/watch?v=LuUhox_C5yg&list=PL1jK3K11NINhvnr7Y3iGu8eLKec72Sl7D
https://pre.empt.dev/
https://0xpat.github.io/
https://www.guitmz.com/
https://www.hackinbo.it/slides/1574880712_How%20to%20write%20malware%20and%20learn%20how%20to%20fight%20it%21.pdf
https://cocomelonc.github.io/
https://0x00sec.org/c/malware/56
https://institute.sektor7.net/red-team-operator-malware-development-essentials (you can find this course leaked online)
https://institute.sektor7.net/rto-maldev-intermediate (you can find this course leaked online)
https://institute.sektor7.net/rto-maldev-adv1 (you can find this course leaked online)
https://captmeelo.com/
https://www.vx-underground.org/
https://google.com/
https://c3rb3ru5d3d53c.github.io/posts/
https://unprotect.it/
https://www.youtube.com/watch?v=xCEKzqLTvqg&list=PL-aDiCywOtNXxR8EGzp773K3sgKQlAlG0
https://github.com/chvancooten/maldev-for-dummies/blob/main/Slides/Malware%20Development%20for%20Dummies%20-%20Hack%20in%20Paris%2030-06-2022%20%26%2001-07-2022.pdf
https://github.com/rootkit-io/awesome-malware-development
