Read Roger Dressler's Dolby Audio Whitepaper (
here). It explains all the Dolby formats in some detail. It is always preferable to read the best official reliable source.
The core of DD+ is DD (p.4, 6).
Dolby TrueHD uses MLP for all the substreams. The core 5.1 in 7.1 presentation is still Dolby TrueHD lossless. Dolby TrueHD can carry a two- and 5.1-channel presentation independently if desired to avoid downmixing (p.11). The DD lossy track is a separate companion track (p.6).