Production of avatars (VRM) and items (VCI) doesn't conclude in Unity itself. You need to use other software to create media, such as images, 3D models and sounds and import them into Unity.

Unity by itself cannot create assets like 3D models, images and music to be used for the production of items (VCIs) and avatars (VRMs).
(Actually, there ARE some ways to create assets in Unity, but we will not discuss them here) Unity is meant to import assets created in other software and combine them to create contents.

To create 3D models, you use modeling feature in 3DCG software.
There are various 3DCG software out there. One of the most popular software for creating avatars (VRMs) and items (VCIs) is Blender.
Blender is an open-source software which can be used for free, yet it is very powerful. As there are many Blender users out there, a lot of tutorials can be found on the Internet.
You can export the model created in Blender as .fbx file, and import it into Unity by drag & dropping it into the Unity assets folder.
You can find a lot of learning materials, such as books and videos. Try searching “Blender” on YouTube and see for your self.

• Create models using Blender and export it as .fbx. Then import it into Unity
• Abundant of learning materials can be found from books to videos
• How do I import models from my 3D app? (Unity manual)
• https://docs.unity3d.com/Manual/HOWTO-exportFBX.html (Unity manual)

You use graphics software to create images.
Photoshop and illustrator are two of the most popular graphics software. The most significant difference between the two is the type of data they use: raster and vector. In raster format, images are manipulated pixel by pixel, while in the vector format, image is manipulated by editing formulas to draw lines.
For the purpose of creating textures for 3D models, such as avatars and items, the graphics software that handles raster format is more appropriate.
So we recommend you to choose software that handles raster format.
Softwares that mainly uses raster format is often referred to as “paint software,” while software that uses vector format is referred to as “draw software”.

• There are basically two types of drawing software
• Draw software uses vector format
• Paint software uses raster format
• Paint software is appropriate for creating textures for 3D models

Here are some example of software that uses raster format.
GIMP the free software, Photoshop the most known premium software, CLIP STUDIO PAINT the popular in Japan, and ibisPaint the smartphone compatible. You create image data in these software and export them as .png, .bmp, etc. Then drag & drop them into Unity's asset folder to import them.
Other image types are also supported, however, .png and .bmp is commonly used as they offer superior image quality.

You need a lot of equipment and software to create music..
Audacity is an easy to use free software that offers basic recording and editing functions.
You can download it from here.
Record, cut and edit sound in the software and export it as .wav. Then drag & drop them into Unity's asset folder to import them.

• Audio file Unity manual