In its early phases, the Marvel Cinematic Universe grounded its stories in realism. Movies like Iron Man and Captain America presented scenarios rooted in our history. While Thor introduced fantastical elements, framing the magic as advanced science. Over time, however, the MCU embraced more and more sci-fi concepts, including aliens, multiverses, and cosmic entities, normalizing these for Joe Audience (read: the general audience member). However, moving into sci-fi does not mean realism is lost altogether. It can still be made to feel real or grounded if it follows certain certain rules.
Soft and hard rules
Soft rules are common in movies like Guardians of the Galaxy and Thor. The former initially leans on sci-fi elements but moves into fantasy with Ego’s introduction. Matter and energy manipulation sounds like science, but could just as easily be a fantasy trope. Similarly, Thor begins by explaining gods and magic as advanced science, but later embraces mythological elements fully, leaning into Asgardians being gods and using magic in Ragnarok. Loki eventually does the same. This continues in Moon Knight and Black Panther, where divine avatars and celestial realms are part of the stories.
Hard rules are seen with Pym Particles in Ant-Man. Initially explained as reducing atomic spaces while retaining mass, these rules are occasionally ignored for the rule of cool, such as when a shrunken tank is light enough to carry as a keychain, while it should have retained it's mass. The particles also allow for time-travel through the Quantum Realm in Avengers Endgame. The realm is fictional, but the rules (as far as theory allows) are not. Endgame introduced the concept of branching timelines, although shows like Ms. Marvel and Loki have leaned into time loops -where changing the past alters the present.
To recap Endgame's rules: altering events in the past creates a new, divergent timeline, leaving the original timeline the traveler is from unchanged. This avoids the grandfather paradox, where someone could erase their own existence by altering the past. Thus, actions taken in the past don’t overwrite the future but instead create alternate realities that coexist. To prevent creating chaotic branches, the Avengers returned the Infinity Stones to when they were taken. However, since then the MCU has had characters run rampant across the multiverse, possibly creating a lot of chaos on the timelines.
Science-f(r)iction
A show that does try to follow hard sci-fi rules as much as possible is The Expanse. Aside from a few fictional elements in the story, it abides to rules rooted in real-life physics: there is no sound in space, artifical gravity comes from acceleration or centrifugional force, radio contact takes longer in relation to the distance it takes light to travel, etc. Even though it limits itself by following these hard rules, the show is very scientifically accurate and feels very realistic. If the science isn't accurate though, it can still be made to feel as such though by using scientific sounding terms to hide behind.
A show that does take a lighter approach, is The Orville . It draws inspiration from Star Trek and uses established sci-fi tropes and rules (bar teleportation) to craft it's own world and history. Similarly, Avatar uses common sci-fi concepts like interstellar travel and resource mining, but undermines it by depicting humans expending vast resources (like rare Dark Matter) on it while there is an energy crisis on Earth. In The Way of Water, humans return to harvest a liquid to counter aging. However, it is shown humans have already mastered cloning and mindtransfer. As such, both trips feel redundant.
Science-fantasy
Fantasy and science-fiction aren't mutually exclusive though, like with Destiny and Star Wars. These franchises feature typical sci-fi elements like space travel and alien species, alongside fantastical tropes like magic (abilities and the Force), dark lords (Savanthun and Darth Vader), and traditional quests (rescue the princess from the tower in A New Hope). As they sacrifice adherence to scientific rules in favor of fantastical elements, but still feature sci-fi tropes, they fall in a category in-between that is neither low (set on Earth like Harry Potter) or high fantasy (set on another world like Dungeons and Dragons).
Lastly, Joe Audience can be eased into both. The MCU has slowly build up it's world over dozens of movies, while Indiana Jones did it over decades. It started with mystical elements, like voodoo and supernatural Christian elements, while it later moved on to sci-fi elements, such as dimensional and time travel. Although this is a shift, at it's core the same humour and extra added elements in chases and problems, like Helena's fiancee chasing them while they're chasing Voller, remain. So to conclude, no matter the genre, everything makes sense if a franchise follows rules and tropes and eases Joe Audience into them.