Katha-2-1-6-yaḥ pūrvaṃ

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

This shows that he who was pointed as the pratyagâtman and Isvara is the âtman of all. Yah ] some seeker after emancipation. Pûrvam ] first. Tapasah ] from the Brahman defined as knowledge, etc. Jâtam ] created or produced, the first-born of Brahman, i.e., Hiranyagarhha. ‘Born before whom’ is explained. Adbhyah pûrvam ] before the five elements, including water; not before water alone; ajâyata ] was born. This first-born who having created the bodies of the Dêvâs, etc., entered the âkâs of the heart of every living thing and is there seated, perceiving sounds, etc., Bhûtêbhih means Bhûtâh or elements in the nature of cause and effect. Who sees him seated with them: who sees thus. See this alone, i.e., Brahman the subject of our present discussion.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda