What is a Buddha? Is the real Buddha the one appearing in a thangka with golden face and sitting in a full lotus position?

That is only a partially real Buddha. In the view of Mahayana, the Nirmanakaya (Emanation Body) and the Sambhogakaya (Bliss Body) are the manifestations of the Buddha in order to liberate ordinary people and bodhisattvas of the first to the tenth bhumi, respectively.   The Nirmanakaya is for the Buddha to communicate with ordinary people. Although Buddha-nature exists within the mind of every sentient being, the Dharmakaya (Truth Body) of the Buddha is rendered powerless to those who have not attained realization and thus must rely on the Nirmanakaya and the Sambhogakaya of the Buddha for guidance to enlightenment. However, neither the Nirmanakaya nor the Sambhogakaya is the true Buddha, only the Dharmakaya, the union of wisdom and compassion, is.

~Depicted from THE RIGHT VIEW - Buddhism—the Definition

Sakyamuni Buddha expounded this principle two thousand years ago. Modern physics also acknowledges the continuous process of small particles arising and ceasing in a moment, in one thousandth of a second or even one ten-thousandth of a second. However, what the Buddha taught is more profound and explicit than modern physics, since the concept of an indivisible moment which is significantly smaller than one thousandth of a second or one ten-thousandth of a second is virtually inconceivable to us. The arising and ceasing that takes place in such a moment is so subtle we cannot begin to comprehend it.

~Depicted from THE FOUR SEALS OF DHARMA - All Composite Phenomena are Impermanent

From a short-term perspective, the significance of releasing tens of thousands of lives from the suffering of death is already self-evident. There is absolutely no comparison between giving a new life and giving money or other objects to a being facing death. Put in another way, if we are about to be killed, would we like someone to rescue us or give us a lot of money? The answer should be obvious. Realistically, what is the use of money for a dead person? In most cases, the relatives and the friends of the deceased do not really know how to use the money left behind to assist the deceased, e.g., to perform phowa. At the juncture of life and death, money loses its purpose. Saving lives is naturally the most important.

~Depicted from THE RIGHT VIEW - Liberating Living Beings

When facing death, all the worldly wealth, knowledge, experience and so forth are rendered useless. Without prior training in preparation for one’s mortality, every person, however prominent his or her status in life is, will panic in the face of death. Many people become extremely pessimistic and despaired, turning gray-faced and gaunt in no time, after being diagnosed with advanced cancer and given the verdict of death by the doctors. Very often their fear is more detrimental to their condition than cancer itself.

~Depicted from THE HANDBOOK'S FOR LIFE JOURNEY - On Death And Rebirth-How to Face Death

The posture that one takes at time of death is quite important. When dying, one should keep the same lying position as that of Sakyamuni Buddha when he went into parinirvana—head to the north, feet south, lying on the right side of the body, placing right hand under the head, pressing the right nostril with the right little finger and breathing through the left nostril. It is said in the scriptures that when lying on the right side of the body, breathing will naturally go through the left nostril. Nevertheless, it is still important to block the right nostril. Whether being Buddhist or not, by taking the right posture when dying, one can be sure not to take rebirth in the three lower realms for the next one or two lives, even without the help of any other transference ritual.

~Depicted from THE HANDBOOK'S FOR LIFE JOURNEY -On Death And Rebirth-How to Face Death

To liberate lives is a common practice frequently performed by many Buddhists. When conducted properly, the resulting merit is boundless. Otherwise, the merit will be greatly diminished. It is therefore very important for us to know the proper way of liberating living beings.

~Depicted from THE RIGHT VIEW - Liberating Living Beings

In sutra, ground is the illusory world we see in front of us, which includes sentient beings, the external world of mountains, rivers, earth, etc., and emptiness; fruition is the attainment of the three resultant bodies of buddhahood—the dharmakaya, sambhogakaya, and nirmanakaya. We ordinary people are not yet buddha, but through practice we can attain buddhahood after three asamkhyeya kalpas. Sentient being is the cause; buddhahood is the result.

~Depicted from GATEWAY TO VAJRAYANA PATH - Vajrayana Terminology

The word mandala is a Sanskrit term that means an encircled universe with a core center.

This mandala appears before all sentient beings during bardo; at the time of death, an experience similar to the condition around the Big Bang in the universe occurs—our eye, ear, nose, tongue, body, mental consciousness, as well alaya consciousness all disappear or temporarily stop operating. The alaya consciousness is the carrier of all impure phenomena; when it disappears, a luminous state emerges. In tantra, this is called the ground luminosity or mother luminosity, the fundamental and original face of all phenomena. In Dzogchen, it is also called the clear light of death, because it cannot appear under normal circumstances, only at the time of death.

~Depicted from GATEWAY TO VAJRAYANA PATH - Vajrayana Terminology

We all know that the Buddha transmitted the Dharma to followers according to their capacity and guided them on the path step by step. The Outer Tantra, having also empowerment, the practice of generation stage and so forth, is part of Vajrayana, but it lacks the view of the Inner Tantra. Hence, practitioners can only visualize themselves as they are in the present world (though sometimes they also visualize themselves as buddha), then visualize the buddha and the yidam in front and recite mantras. The Outer Tantra offers many ways to recite mantras; it is of the view that one can attain buddhahood by way of mantra recitation.

~Depicted from GATEWAY TO VAJRAYANA PATH - The Parable of a Black Snake