And God said, “Let there be an expanse (rakiya) in the midst of the waters, and let it separate the waters from the waters.”
Genesis 1:6

Rakiya doesn't necessarily mean a solid surface, but anything stretched out and open, even empty space. The English word "firmament" tricks people into thinking it's intended to be something "firm", but Genesis wasn't written in English.

The expanse is an empty space between the waters. If it was meant to imply a literal, physical barrier, then there should be no air between the two bodies of water. The existence of air between them implies that the air itself is the rakiya.

"In the midst of the waters" means between the waters, not in the middle of the waters. The waters above the expanse could be the clouds or the vacuum above the atmosphere, which looks from the ground as if it is an expanse of water, or something that no longer exists.