For Strongholds, you can use this:
For EndCities you can try this tool:
There is those mods but if you after a command (or mods that do the commands before 1.16) then you have /locatebiome and mods with that title. But for survival use those mods TileEntity provided are more what your looking for.
Forum Thread Maintainer: Fabric Project, Legacy Fabric, Power API, Rift/Fabric/Forge 1.13 to 1.16,
-Niche Community Content Finder, -Youtuber, -Modpack/Map Maker, -Duck
Major Forums I Maintain:
Fabric/Forge 1.16: https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/minecraft-mods/mods-discussion/3004971-fabric-1-16-snapshot-discussion-thread-for-20w06a
Fabric/Forge 1.15: https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/minecraft-mods/mods-discussion/2988502-fabric-19w34a-to-1-15-2-rel-snapshot-discussion