A Scriptable Render Pipeline (SRP) allows you to control rendering via C# scripts, giving you a very high degree of customization.
Unity provides two prebuilt Scriptable Render Pipelines: the High Definition Render Pipeline (HDRP) and the Universal Render Pipeline (URP). Both of these offer extensive customization options. If you need even more control over your rendering pipeline, you can create your own custom SRP.
You can create your own custom SRP from scratch, or you can modify URP or HDRP to suit your needs.
For more information on custom SRPs, or on the technology that underpins Unity’s prebuilt SRPs, see the SRP Core package documentation microsite.