Subscribe to any mod/map that you wish here (it will auto-download in-game or when idling on Steam depending on your Steam settings); https://steamhost.cn/steamcommunity_com/app/285190/workshop/ , then when you start the game make a custom solo/online lobby, select the custom mod or map that you wish to play with and then start the match.

Also some maps will only be available in certain game modes and some others won't show up in game because they weren't updated after the patch that introduced Annihilation. Click the minimap image in the game lobby to change map. There are drop down menus under the minimap image that let you select game mode and mods. If you don't see the map you want in the list and you know you downloaded it then try changing the game mode and look again. In game you can go to the community tab and then workshop to see what maps and mods you have installed.

Subscribe via the steam workshop outside of the game (click workshop, which is visible somewhere when you have DOW3 selected on your games library) or subscribe to a map or mod via the in-game 'Community' > 'Workshop' tabs.