Tafsir of Ash-Shams 91:5

"And the heaven and what built it" means: and He who built it. The preference for ma (what/that which) over man (who) is intended to signify the attribute, exalting it [the act of building] beyond what preceded in the verse "and what He begot," as if it were said: "And the All-Powerful, of Great Stature, who built it."

The act of building them points to His existence and the perfection of His power. The intended meaning of "building" here is bringing them into existence in such a manner that they signify that [existence and power], and serve as evidence for Him. This is more appropriate than interpreting it as "its builder," because it [the use of ma] draws attention to the purpose behind the building. The same applies to the speech regarding His saying, the Exalted: [...]