To explore the maps, we'll have to create a custom game. To do so, launch the game and navigate to:
Play > Multiplayer > Custom Game > Local > Create > Create New Playlist.

Click the Create New Playlist button, select normal mode, and give your playlist a name.

Then, Click Add a Match.

Under Match One, select the map you'd like to play. Then, next to Change Game Mode Settings, change the Action Phase Duration to 600 seconds. You can also add time to the Prep phase duration if you'd like additional time to run around inside as Defense, but 600 seconds should be plenty.

I also like to change the Number of rounds in Match Settings to 1, so that I can quickly go from one map to the next.

You will have to configure a Match for each map you'd like to explore, but there's a way to make this go quickly. In the list of buttons under Match One there's a "Duplicate Match" button. Click this, and all of your settings for the Match will be duplicated into Match 2. All you have to do for Match 2 is change it to the map you'd like to explore.

You can do this for up to 15 maps, so I'd recommend doing it for the pool of competitive maps (which you can view here: https://rainbow6.ubisoft.com/siege/en-us/news/152-299564-16/changes-to-map-rotations-in-season-3 ).

To save your maps, scroll to the bottom and click "Save Playlist."

Once you're ready to start exploring, click the playlist you'd like to play on and click Create as Player. I recomend pressing Space to change to the Orange Team (which gets Defense,) because you can look through cameras to see their locations and run around during the Prep phase. I also recommend picking a shotgun operator, so you can breach through walls/trapdoors and see where those paths lead.

Bonus tips:
*You can select what map you'd like to play while in the game lobby by pressing Alt.
*r6maps.com is an excellent resource for learning map callouts and camera locations as well! If you have a multimonitor setup, I would advise having the site open on another monitor while you're learning the map.