From Supergirl Special #1, December 2023. Scan by Daughter of Krypton.
The finish line banner is in Doyle Kryptonian and is written in the Doyle Kryptonian font:

To date, there have been three versions of the Kryptonian language used in Supergirl comic books.

The 1970s-1980s comics books

An evolved language, complete but lost

Writing and language created by E. Nelson Bridwell and Al Turniansky:

• Alphabet
• Historical Note
• Grammatical Notes
• Glossary

The 2000-2020 comic books

Transliteration Font

A Kryptonian transliteration font was created by Georg Brewer for DC Direct products and for use in DC comic books.  This font was also used in Smallville Supergirl, and other DC-based television shows.

Bottle City of Kandor English/Kryptonian Transliteration Guide
Touch for full-size image

Freehand writing guide:

2020-2023

Kryptonian Language

Beginning in 2003, linguist Darren Doyle started a project to create a full-fledged, real and complete Kryptonian language and writing system. His Kryptonian language incorporated vocabulary and grammar from the earlier Bridwell/Turniansky system, and expanded the Brewer transliteration font for his newer complete Kryptonian language.

Doyle's (basically) fan-created language and writing system has now become canonical: His constructed spoken Kryptonian language was utilized by the creative teams on both the Supergirl and the Superman & Lois live-action television shows, spoken by actors in both series.

His unique language and font have also been used in DC's Supergirl comics (see above). 

For more information on Darren Doyle's Kryptonian Language Project, please visit kryptonian.info —this is now a canonical language and writing system!

In short, both the Brewer Kryptonian Transliteration/Substitution font and the Doyle Kryptonian Language have been utilized by DC. When you see Kryptonian, it could be transliterated English (using the Georg Brewer Kryptonian font); or it could be Doyle Kryptonian (using Doyle's Kryptonian font). One way to tell us by discerning which of the different but similar font designs is being used.

2025

The Return of Kryptonese

As a result of DC's "All In" event, Supergirl, Superman, and other Kryptonians are no longer speaking a transliterated "Kryptonian" but have returned to the 1970s-1980s (Bronze Age) "Kryptonese" of E. Nelson Bridwell and Al Turniansky.

About the Kryptonian languages: here, STTA, Forum