Did you have a killer game? Did you forget to clip an epic moment? Or do you simply want to figure out, definitively, whether that Ela from your last game was cheating or not?
You should utilize Match Replay, the in-client way to watch every perspective from your most recent match.
How to use Match Replay
It's very simple to use Match Replay in "Rainbow Six Siege". First, navigate to your "Watch" tab. You'll need to be in a party alone to view any of your replay files. Only your most recent ten replays are saved.
Periodically, "Rainbow Six" esports broadcasts will also be uploaded to Ubisoft's website, if you'd like to watch your favorite teams. The link above also has instructions on how to download the Match Replays.
Once you're in the Match Replay, the controls are a bit confusing. Your best bet is to use a console controller plugged into your PC, like many professional "Siege" observers do.
If you're forced to use your keyboard, here are the basic controls:
Cycle Players: Q (Left) / E (Right)
Follow Players: 1 – 0 (numbers)
Play / Pause: Spacebar
Select Speed: K
Select Round: U
Toggle View Mode: C
Toggle Control Bar: R
Zoom In: Page Up
Zoom Out: Page Down
From there, you can do anything you want with the Match Replay system -- create tons of compelling content, watch your favorite games back from multiple perspectives, or check out your favorite player's perspective.