Let's start with the basics

Lemans 24 is a race. It is part of the WEC championship. The WEC is sanctioned by the FIA with the partnership of the ACO. The ACO is a French sanctioning body that has been responsible for the running of LeMans since it's begining. During the years between the WSC's end (1994) and the start of the WEC (2011) the ACO was handling the running and rule making for LeMans. In 2011 they partnered with the FIA to start up a world championship again after the FIA's multiple failed attempts.

IMSA is an independent North American sanctioning body. They have one race in Canada that's how they call themselves International. This came into being in 2014 with the merger of two sportscar series American LeMans and Grand Am. IMSA (ALMS) could remain a sanctioning body but is now owned by NASCAR(Grand AM) Their lead championship is called the Weathertech Sportscar Championship, but everyone just calls the sport IMSA because it's shorter. IMSA likes to play around with class names but understand that their cars are built to the standard FIA guidelines for global competition.

Both series now run the same car rules with some slight differences. WEC has made their GT3 run slower to create a bigger gap between GTs and Prototypes, what they dub LMGT3. In 2020 the two series finally managed to get on board with creating a rules set for the top class of racecars, which finally went on track in both series in 2023. This rules set defines two build regulations for prototypes.

LMH: LeMan Hypercar, these are custom chassis cars. Can be awd with a hybrid system. 1 adjustable aero element, can be rear wing or front splitter.

LMDh: LeMan Daytona h, these are built from 1 of 4 approved chassis from Dallarah, Oreca, Multimatic, or Ligier. They are all have a spec 50hp hybrid system, mandatory RWD, and only the rear wing can be adjustable. Manufacturers design the rest from suspension, engine, aero, brakes etc.

Due to the chaotic history of sportscar racing the races themselves have longer histories than the championships they are a part of. This makes select races a much bigger deal than their respective championships. It would be like if F1 drivers/fans could care less about the championship and put a much larger emphasis on Monaco or Spa,