[The information in this post is based on a university history course about water use and hydro-engineering]
Depending on the type of aqueduct we're talking about, aqueducts are more or less suited to be district tiles. The most famous aqueducts are the Roman variety- they stretched for miles and relied on incredible precision of engineering to function. The purpose of Roman aqueducts was to provide clean water to cities. River water is unclean at best, often polluted with sewage or animal excrement and riddled with disease. As such, it was largely unfit for drinking and with cities of tens of thousands, and the various needs for water beyond drinking, wells were impractical. Roman engineers explored nearby terrain in search of natural springs- areas where underground aquifers well up to the surface. Due to the sterile environment underground, these springs were inherently clean and free of bacteria or contaminants. A major constraint however was that the spring had to be high enough above the city and that the aqueduct itself couldn't be lower than the city (except in certain exceptions). Roman engineers meticulously measured the exact elevation between the city and the spring source with very low-tech tools, like a measured stick and chorobates, which is a very heavy wooden table with a portion of water and two eye holes. The table was supported by rocks or debris until the water was level. One engineer would then peer through the two holes to where another engineer was holding the measured pole. The height of the table is subtracted from the observed height on the pole and the result is recorded as the difference in elevation between the two points. As you can imagine, this is a very difficult method to use accurately.
In the city of Nimes, the only spring that could be located was 50 km away (around 31 miles). In addition, the spring was only 17 m (55' 9") above the city. That meant the average gradient of the aqueduct could not be more than 17:50000, or about 1 mm every 3 m (~1/25th of an inch every ten feet). Despite this, they successfully completed the aqueduct and it functioned for hundreds of years.
There are a number of errors with how aqueducts are presented however: aqueducts were only built above ground when they needed to bridge a valley or gulley (especially rivers). In instances where the gap was too far or deep to bridge, they'd employ piping in a reverse siphon to drop to the valley floor and then back up the other side. Regardless, they attempted to avoid building aqueducts above ground at all costs: not only was it more expensive and time consuming, it made them vulnerable. As the only source of clean water for the entire city, aqueducts were incredibly vulnerable to sabotage, and they were often buried in the ground. Access ports for maintenance (sediments in the water would eventually collect in limestone sheets in the bottom of aqueduct channels. These sheets would be harvested periodically and used as tombstones among other purposes) were placed at set distances and would be hidden with vegetation.
The other type of aqueduct which is present in Civ is the variety that were built in the middle east after the fall of the Roman empire. Modeled after earlier Roman examples, they often focused on drawing water directly from rivers (usually rivers that the city was settled on) using Noria. Civ 6 aqueducts that are built onto rivers have these Noria rendered (they're large water wheels that are powered by the movement of the river. They lift water and drop it into the top of the trough of the aqueduct). This sort of aqueduct does not provide clean water. This water was only fit for use in public baths or for any purposes where water power might be useful in the city.
Regardless of the purpose however, aqueducts are relatively small, though long. Most aren't more than a few meters across, so the idea that they even occupy an entire tile is a little silly. It makes (a bit) of sense that you can't build them through districts, but honestly I don't even think they should obstruct normal improvement placement (it's completely reasonable to have an aqueduct go through farmland. The impact would be negligible at best). However, it should be possible to pillage an aqueduct while sieging a city.
