How to build a retaining wall for back yard?

https://www.thisoldhouse.com/how-to/how-to-build-retaining-wall

I worked for the government, in the SBA department, that's small business administration. I worked in California on the floods and fires, A retaining wall has to be as deep as it is high. Ex. 3 ft high, 3 ft underground. You need a mason for this. If this is not what you want, it's not a retaining wall.

I'm also inclined to think this is not a retaining wall. What is the difference in elevation between the bottom and top? What sort of budget do you have? What sort of aesthetic feel do you want? What do you want to use the area below the wall and above(the top) the wall for?

We would like to use the whole space. Maybe concrete to have an outdoor kitchen. Budget not much we need classes DIY.

Thanks for you response