Most browsers (Chrome, Edge, Opera) ship with this key pre-installed. Brave is built on Chromium (the same engine as Google Chrome). However, Brave removes many Google "phone-home" features to protect your privacy.
Here is the issue: It is not open source. google widevine brave
Open Netflix again. You should be streaming in 1080p (or 4K, depending on your plan) immediately. While the fix above gets you streaming, you should know the limits. Due to licensing restrictions from Google and Netflix, Brave (like Firefox) is generally limited to 720p or 1080p. Most browsers (Chrome, Edge, Opera) ship with this