The shape and picture quality of Star Trek as television technology changed—from the original 4:3 screen to modern streaming. The key thing is that aspect ratio and resolution are different: 4:3 or widescreen describes the picture’s shape; standard definition, HD, and 4K describe its detail.

Era and series	How it was made	What that means on a modern TV
The Original Series (1966–69)	Shot on 35mm film in 4:3. It was broadcast on analog color TV, which had much less detail than HD. The film held more detail than the broadcast showed.	Later HD editions could be made by scanning the original film. The show stays 4:3, so it appears with black bars on the left and right of a widescreen TV.
The Next Generation, Deep Space Nine, Voyager (1987–2001)	Still filmed on 35mm and made in 4:3, but episodes were edited and effects were assembled on standard-definition videotape.	TNG was rebuilt in HD from the film negatives, with effects recomposed for the new masters. DS9 and Voyager have not had the same complete HD reconstruction, so streaming versions may look softer.
Enterprise (2001–05)	The first Trek series made for a widescreen 16:9 picture. Its first three seasons were shot on film but finished in HD; season four switched to digital cameras.	It was designed for widescreen viewing and was already prepared for HD, rather than needing the extensive reconstruction TNG required.
Discovery and Picard (2017–23)	Made with digital cinema cameras and modern digital effects. Picard uses a broad, cinematic 2.39:1 frame.	On a normal 16:9 TV, the very wide picture has black bars above and below. Paramount+ lists Picard among its Dolby Vision titles on supported devices, but it has generally streamed in 1080p HD, not 4K.

So the visual change was not simply “old shows were 4:3, new shows are HD.” TOS and TNG were filmed on high-quality film, but their original TV versions were limited by standard-definition broadcast and video editing. TNG could be brought back from its film sources; DS9 and Voyager are harder because much of their finished editing and effects work survives in standard definition.

On Paramount+, the format also depends on the title, your subscription, and your device. Paramount says supported titles may stream in HD, 4K, HDR10, or Dolby Vision, and that available formats are marked on the title page.

A quick guide to the black bars:

4:3 shows such as TOS and TNG: bars on the left and right.
2.39:1 shows such as Picard: bars at the top and bottom.
The bars preserve the original picture; stretching or cropping would change it.

Home video adds another important layer. Regular DVD was not high definition. It stored a digital copy of the standard-definition picture. HD DVD and Blu-ray were the high-definition disc formats.

Home format	What you got with Star Trek
VHS	The first widely available home copies were analog, standard-definition tapes. A cassette usually held only a few episodes, so collecting a series took many tapes.
DVD	In the late 1990s and early 2000s, the shows became available on digital discs. The picture was still standard definition, even though it was digital; a modern TV may enlarge it, but that doesn’t restore detail that wasn’t on the disc.
HD DVD	A short-lived high-definition disc format. In 2007, The Original Series was released in HD DVD/DVD combo sets. The HD DVD side offered a remastered high-definition picture; the DVD side was for ordinary DVD players.
Blu-ray	Blu-ray became the lasting high-definition disc format. The Original Series came out in HD on Blu-ray in 2009. The Next Generation followed in 2012, after CBS rebuilt the episodes from film and recomposed their visual effects.
Streaming	Streaming makes shows convenient to watch, but the quality depends on the particular title, plan, device, and internet connection. Paramount+ offers select titles in 4K or HDR formats; that does not mean every Trek series is available in 4K.

The big surprise is that an HD release doesn’t necessarily mean the show was originally made in HD. TOS and TNG were shot on film, which gave the studios a detailed source to work from later. TNG needed a major reconstruction because its original editing and effects were done on standard-definition video. By contrast, DS9 and Voyager were also shot on film, but they have not received the same complete HD rebuilding.

For your examples: VHS → DVD mostly improved convenience and durability; HD DVD/Blu-ray brought a genuine increase in picture detail; and Paramount+ can add modern streaming features such as HDR for selected shows. Picard is widescreen and available with Dolby Vision on supported setups, though its streaming picture has generally been 1080p HD rather than 4K.