The Zwicky Transient Facility (ZTF) conducts a wide-field time-domain survey using the Samuel Oschin 48-inch Schmidt Telescope at Palomar Observatory. The initial observations began in March 2018, and since the first data release, new versions of the data have been published regularly, approximately every two to six months. ZTF Data Release 21 (DR21) includes observational data from March 17, 2018, to February 29, 2024, spanning about 71 months. This encompasses both public and private survey projects, with private survey observations concluding on October 31, 2022. The camera used for observations consists of 16 CCDs, each divided into 4 readout quadrants, resulting in 64 CCD-quadrant images. Each quadrant covers an area of approximately 0.854° by 0.854° on the sky. Observations are primarily conducted in the g, r, and i bands, with i-band data originating from private survey time. The main exposure time is 30 seconds, although private surveys also include exposures of 60, 90, 120, 240, and 300 seconds. Data products include raw CCD images, calibrated science images, reference images, difference images, catalogs, and light curves. The processing pipeline is managed by the ZTF Science Data System (ZSDS) at IPAC/Caltech and includes astrometric and photometric calibration, image co-addition, and difference imaging. ZTF DR21 contains approximately 56.3 million single-exposure images, 176 thousand co-added images, 860 billion detected sources, and about 4.89 billion light curves. Data quality has undergone basic automated checks but may still contain low-quality data affected by factors such as clouds, atmospheric transparency variations, and moonlight contamination. Users can filter data using quality flags. Calibrated ZTF photometry may exhibit systematic biases of up to 0.025 magnitudes for bright sources (brighter than 15.5 magnitude), and these biases are corrected in the light curve data. ZTF DR21 is a valuable resource for time-domain astronomy research, covering a large portion of the northern sky, with a long time span and moderate depth across multiple bands. The mirrored data in this release includes only light curve data. Files are organized in subdirectories by observational field ID, with each field covering an area of approximately 7° by 7°. The DR21 light curve data spans 1,181 fields, with a total of 176,218 parquet files. These files can be read using libraries such as Pandas, pyarrow, or Dask. For more detailed instructions on using the light curve files, refer to section 12.c of the ZTF DR21 documentation. The documentation can be accessed at the following link: https://irsa.ipac.caltech.edu/data/ZTF/docs/releases/dr21/ztf_release_notes_dr21.pdf.