These files appear as lens.lns files in the cache folder of the Snap Camera desktop application, which officially ceased operations on January 25, 2023. It appears to be a proprietary but uniform format used by Snap Inc. to publish and deliver its AR Lenses to its own applications.
Not much was known about this file format until now. All findings in this document are based on reverse engineering and trial and error.
The file itself consists of a Main Header, a File Allocation Table (FAT), a very small Sub Header, and BLOB of Compressed Data using the Zstandard algorithm.
Zstandard, commonly known by the name of its reference implementation zstd, is a lossless data compression algorithm developed by Yann Collet at Facebook. The zstd source code is dual-licensed under BSD and GPLv2.
The File Allocation Table addresses the uncompressed data and varies in size. The exact size of the FAT is specified in the last 4 bytes of the Main Header.