The CMIP6 Special Issue in GMD includes an overview paper on the CMIP6 design and organization (Eyring et al. 2016, doi), contributions from CMIP6-endorsed MIPs and descriptions of the forcing data sets: https://www.geosci-model-dev.net/special_issue590.html . Detailed information are available at the PCMDI CMIP6 website https://pcmdi.llnl.gov/CMIP6/.
Summary table of currently available CMIP6 data https://pcmdi.llnl.gov/CMIP6/ArchiveStatistics/esgf_data_holdings/
The complete archive of CMIP6 output is accessible from any one of the following portals:
If using the DKRZ system, the various possibilities to acceess CMIP6 output directly are described here: