Upanishads prescribe the method of upasana – meditation – with the following principle:
Tam yatha yatha upasate, sa tathaiva bhavati.
As one adores, so does one become. The impression created by continuous thinking, wanting and adoring whatever objective may be in one’s mind – that concretises itself into a form and presents itself before oneself. This is just like kids are given a role model like Hanumanji to follow so that they think about him, adore him continuously to gain the qualities of the role model you put before them.
Meditation, in the sense of Upasana, is to remove all conditioning factors which make you feel you are finite, limited by Body, Mind and Intellect. This is possible only if you can move above BMI and defy the limitation of space and time. Since our experience even in deep sleep is limited to non-apprehension of infinite nature of the Self, the universal concepts of Gross, Subtle and Causal bodies are given in Ishwara to do Upasana (meditate) upon.
To summarise, Iswara is the unmanifested form of Brahman with Maya as Upadhi. The unmanisfested or avayktam is manifested in the form of Hiranyagarbha or the Total subtle body. The world is created by Iswara and the creation happens in Hiranyagarbha. This creation is manifested externally through the Total gross body or Virat. You do the Upasana on these concepts, thru your limited Gross (Viswa) /S (Taijasa) /C (Prajna) bodies to overcome the limitations you feel at individual level.
Best source that gives details of these concepts, as I found them, are in Purusha Sooktam- The Universal Person, with commentary by Gurudev. You will find some elaboration also in mission books like Viveka Choodamani (under verse 283) and Self Unfoldment (book used in study class).
Hope this helps.