No not the same. They come from the same developer however:
AC - Very much a platform for modders, comes with ok base level of content (feeling a bit dated now), however what makes AC shine is the vast amount of community produced mods/tracks/cars that effectively cover pretty much every facet of motorsport, and bring the game very much right up to date. Some mod quality can be very questionable and can range from janky AF to professional developer quality, you just have to go by community reviews. Online racing can be a bit of a minefield and its pretty much necessary to be part of a league/community to get "good" racing.
ACC - Solely focuses on GT racing, fully licensed from SRO motorsports. Very good selection of tracks with all DLC. Cars are primarily GT3 however they have subsequently added GT4 and there are some single make cup cars as well like Porsche and Lambo. ACC in my opinion is a step forward in immersion, the sound and feel of the game are such an amazing experience. Though going by what some real world drivers have commented I think to some extent real world driving physics has taken a slight hit to enable the overall atmosphere of the game. Online is hit and miss on this, but there are a couple of projects to help improve this like LFM and PitSkill.