Strongholds can generate at any Y level, mostly underground, and prefer to generate in biomes above sea level. According to the Minecraft Wiki, there is no fixed depth to find strongholds, but its best to search for them between the base level (Y=0) and the Bedrock level (Y=-64) of the world. However, a Reddit post from 2019 suggests that strongholds can generate at any Y-level. Another post on gaming.stackexchange.com from 2013 confirms that there is no level range that strongholds must spawn at. Therefore, while its best to search for strongholds between Y=0 and Y=-64, they can generate at any Y-level.