A segment VAG vehicles:
A1 Golf, Typ 17, (sold here as the Rabbit) went from 1975 through 1984 for us (it came out in '74 in Europe, and was still being built in South Africa until around 2003... also continued for us in convertible form, renamed Cabriolet in '85, through 1993). The first generation Jetta was a sedan version of this car, came out in '81 for us, through 1984.
A2 Golf, Typ 19, along with the Jetta version, came out for us in '85, and continued through 1992.
A3 Golf and Jetta, Typ 1H, came out in 1993, but we didn't get any until 1994. Went through the first have of the 1999 model year. The convertible Golf, renamed Cabrio in '95, ran on this platform until the end in 2002.
A4 Golf and Jetta, Typ 1J, came to us for the latter half of the 1999 model year. Jetta remained on this platform in sedan version until middle 2005. Golf remained on this platform through 2006. This period also got us a wagon version, badged and trimmed as a Jetta, from 2001-05 (Canada got the wagon for the 2006 model year, too).
A5 Golf, Typ 1K, which came out for us in 2007 was once again renamed Rabbit. The Jetta sedan remained a sedan version of a Golf through 2010. The wagon remained on this platform through 2014.
A6 Golf, Typ 5K, once again called a Golf for us, came along in 2010. Continued though 2014.
A7 Golf, Typ 5Q, came along in 2015, and will be the last Golf we will likely get.
The Jetta SEDAN went from a "Golf with a trunk" shared platform to its own platform, code named NCS (New Compact Sedan) in 2011, and carried that way through 2018. While the Golf in hatchback and wagon trim, however they were named, continued along their merry way, the NCS is a completely different car. It is a Typ 5C, and is what the Beetle uses as its underpinnings upon returning in 2012 (the 1998- New Beetle is on the A4 platform).
There is a new 7th generation Jetta sedan that came out in 2019, but it came along post Dieselgate, so no diesel versions are available for us, so we don't hear much about them here. But it is different yet.
Much of the newer cars in the A segment are all based on the MQB arrangement, so they share a lot of parts as well as some major components like engines and transmissions.
Note: the "Typ" codes (Type) are carried over to the part numbering system VAG has been using since anyone reading this was probably alive. Those two digits start off many part numbers and can at least give a clue as to the fitment of the part. But VAG will often use a part on various cars, so a car that is a 1K may have a part that is 3C or something. Engines and transmissions (and in some cases differentials) can have their own part number prefix.