In Beraishis, the posuk often says ויהי כן. But, for example, regarding Day 4, after Hashem says "there should be luminaries" (verses 14 and 15) then it says ויהי כן (verse 15), and then it says Hashem MADE them (verse 16) So what do the words mean?
ויהי כן is translated as "and it was so". That is, It was as Hashem had decreed and the details are about to follow in the next pesukim. This follows the general methodology ot the Torah of a general statement followed by the relevant details. I explain this regarding the "two" creation narratives in Differences between Genesis 1 and 2. This principal is followed within the parsha of creation. We see this elsewhere in the creation pesukim. For example, Breishis 1:11
יא וַיֹּאמֶר אֱלֹהִים תַּדְשֵׁא הָאָרֶץ דֶּשֶׁא עֵשֶׂב מַזְרִיעַ זֶרַע עֵץ פְּרִי עֹשֶׂה פְּרִי לְמִינוֹ אֲשֶׁר זַרְעוֹ בוֹ עַל הָאָרֶץ וַיְהִי כֵן:
11 And God said, "Let the earth sprout vegetation, seed yielding herbs and fruit trees producing fruit according to its kind in which its seed is found, on the earth," and it was so.
יב וַתּוֹצֵא הָאָרֶץ דֶּשֶׁא עֵשֶׂב מַזְרִיעַ זֶרַע לְמִינֵהוּ וְעֵץ עֹשֶׂה פְּרִי אֲשֶׁר זַרְעוֹ בוֹ לְמִינֵהוּ וַיַּרְא אֱלֹהִים כִּי טוֹב:
12 And the earth gave forth vegetation, seed yielding herbs according to its kind, and trees producing fruit, in which its seed is found, according to its kind, and God saw that it was good.