One character in the Eternals said something along the lines of, “Dad, this is Superman. I saw him on TV!”
Phastos commented to Ikaris: “Should we start calling you Clark now?”
Everyone in the theater—except for those who just emerged from a melted iceberg—understood the reference.
Small references, such as name-dropping, do not rise to the level of violating another company’s copyright.
So, for example, if a character in a Warner/DCU movie had said to Darkseid—“Who are you? Emperor Palpatine?”—they could have gotten away with it.
Simply mentioning the name of a known character is not enough to constitute a violation of copyright law.
Thanks for reading: How were Marvel allowed to mention Superman in Eternals?, Sorry, my English is bad:)