What does each symbol of Bouddhanath Stupa represent?

Bouddhanath Stupa is one of the greatest Tibetan stupas in Nepal. Every year, most of Tibetan Bouddhists come to Nepal for visiting it. They stay nearby this stupa or at the Gumba as well. They engage in meditation activities with others at the presence of their Guru (meditation teacher).

The Bouddhanath Stupa is rich in symbolism as every figure refers to the enlightenment. For more information, you can go through the following bottom-to-top symbol details.

Three Steps: It is the base of the Bouddhanath stupa. These platforms seem as the three different steps, but their sizes are in decreasing style. However, these platforms represent the symbol of Earth from where the song of devotees can listen carefully who are praying below it. They also symbolize the three jewels such as Buddha, Dharma, and Sangha.
Mandala: It symbolizes the physical representation of Lord Buddha in order to display the transition of the spiritual journey.
Two Plinths: These plinths are created in the shape of a circular. They are the strong base for supporting the Bouddhanath Stupa. They depict the symbol of water.
Dome: It is a rounded vault forming the roof of the Bouddhanath Stupa. It symbolizes the Buddha’s chest or the seven elements of enlightenment such as mindfulness, discrimination, exertion, joy, pliancy, Samadhi, and Equanimity.
Harmika: It represents the eight different noble paths. They are:

Right Understanding
Right View
Right Speech
Right Livelihood
Right Action
Right meditative Concentration
Right Effort
Right Mindfulness

Symbol of Nose: It is a sign of Nirvana. It means a path for reaching enlightenment through meditation.
Eight Eyes: These symbols of eight eyes represent the eyes of Lord Buddha. The Bouddhanath stupa has four sides of the wall and each side contains two eyes. It means Buddha can see everywhere and no one can cheat him.
Thirteen Steps: These steps are designed just above the symbols of Eyes. They look like the thirteen different stairs in the shape of a triangle. As a result, the stairs stand for the thirteen different steps for reaching enlightenment while the triangle refers to the symbol of fire.
Lotus: Just above the thirteen steps, there is the picture of Lotus. This figure shows the signs of purity, self-development, and enlightenment as well.
Prayer Flags: These flags are hung from top to bottom and cover the whole parts of Bouddhanath Stupa. They are of five different colors such as red, white, yellow, green, and blue. All these different colors represent different symbols like love and compassion, Purity and prosperity, wealth and beauty, peace and protection, and knowledge and enlightenment respectively.
Umbrella: It is a protector of Lord Buddha, Dharma, and Sangha. Just above it, there is a glided canopy which is a symbol of air.
Pinnacle: It is a symbol of enlightenment.

Source: https://www.academia.edu/8543144/Boudhanath_Stupa_Reflections_on_Living_Architecture