This is explained in several sutras, shastras and commentaries: The form kayas (nirmana- and sambhoga-) and the formless kaya (dharma-) arise based on the two accumulations of merit (punya) and wisdom (jñana). The form kayas arise depending on the collection merit. This is referred to in Vimalakirti sutra, the Large Prajña Paramita sutra, and other sutras, in the context of the major and minor marks of a buddha's body.
It is not different from the appearance of your own body/form, which has arisen depending on your accumulation of merit and wisdom, in your many previous lives. Your light radiating form in the heavenly realms corresponds to the Sambhogakaya.
How does a Buddha become a Sambhogakaya?
Re: How does a Buddha become a Sambhogakaya?
svaha
"All human beings are born free and equal in dignity and rights.
They are endowed with reason and conscience and should act towards one another in a spirit of brotherhood.
Sarvē mānavāḥ svatantrāḥ samutpannāḥ vartantē api ca, gauravadr̥śā adhikāradr̥śā ca samānāḥ ēva vartantē. Ētē sarvē cētanā-tarka-śaktibhyāṁ susampannāḥ santi. Api ca, sarvē’pi bandhutva-bhāvanayā parasparaṁ vyavaharantu."
Universal Declaration of Human Rights, Article 1. (in english and sanskrit)
"All human beings are born free and equal in dignity and rights.
They are endowed with reason and conscience and should act towards one another in a spirit of brotherhood.
Sarvē mānavāḥ svatantrāḥ samutpannāḥ vartantē api ca, gauravadr̥śā adhikāradr̥śā ca samānāḥ ēva vartantē. Ētē sarvē cētanā-tarka-śaktibhyāṁ susampannāḥ santi. Api ca, sarvē’pi bandhutva-bhāvanayā parasparaṁ vyavaharantu."
Universal Declaration of Human Rights, Article 1. (in english and sanskrit)