XLSX is awkward for scripts, databases and import tools that expect plain text. JSONL can be streamed and appended to one record at a time. XLSX is at home in spreadsheets with formulas, formatting and several sheets; JSONL is the usual choice for training data, logs and record streams processed line by line.
One XLSX can produce several JSONL files (one record per line); when it does, they are offered together as a ZIP download.
Each record is written on its own line with no enclosing array. A workbook or database with several tables gives one file per table.