Try to write some shadertoy projects.
Firtst, reflection. (https://www.shadertoy.com/view/wXfyDM)
Second, water wave. (https://www.shadertoy.com/view/MttfW8)
Third, ray tracing scene.
Later,FSR,SGSR2(https://zhuanlan.zhihu.com/p/8980985121),SSR,SSRTGI, SS[Specular, GI, AO]...
SSGI(https://www.shadertoy.com/view/dsGBzW)
DDGI
Nvidia SSRTGI (not free)
Forth, render technique. (Probe)