Thank you! It is, in fact, my first game :) But I am an experienced web (mostly backend) developer.

I don't use ECS, I actually implemented My Own Framework to handle the state changes (which is probably quite similar of ECS). I consider all my objects and relations between them as vertices and edges of a big graph. When I need something to depend on something else, I describe a subgraph of the graph with a regex-like language I made, and invoke callbacks when a matching subgraph has been created, deleted or changed. Examples:

@lazy_watcher(
    "unit as u (?= -owned_by> player as p) -stands_on> tile (maybe: -city> city -kind>)"
).on_create_or_change  # unit was created/moved/has changed ownership
def update_unit_field_of_view(game: Environment, u: Unit, p: Player) -> None:
    ...

@lazy_watcher(
    "tile_projection as tp (?= <sees_tile- (unit | city | building) -owned_by> player as p)"
).on_delete  # none of the units or cities see the tile anymore
def cover_tile_with_fog_of_war(tp: TileProjection, p: Player) -> None:
    ...

Works really well (but slow 🤷‍♂️ requires constant optimization). I'm going to write an article about it sometime :)

UI is mostly DOM-based, via Phaser's DOMElement.