BibTeX is the de facto bibliography format for LaTeX since 1985, plain-text entries like @article{key, author={...}, title={...}, journal={...}, year={2024}}. Used by every academic publisher's LaTeX template and supported as an import/export format by Zotero, Mendeley, EndNote, and Papers. Strengths: trivially diff-able in git, scriptable, tooling-rich. Weakness: there's no single canonical spec, so different parsers handle edge cases (special characters, cross-references, @string macros) inconsistently.
How to open
Any text editor (the format is plain text). Reference managers like Zotero, Mendeley, JabRef, BibDesk all read and write it natively.