Conclusions:
1-Every time that I toggled the extension on/off in any of my profiles, I have to relaunch the browser.
2-Using a certain profile and with Widevine OFF in brave://settings/extensions, when I try to access for the first time to a website that requires Widevine I will be prompted to allow the installation and to run Widevine. After that, with the same profile and if I select Do not ask again, I will never be prompted to allow the installation and to run Widevine on websites that requires it.
3-If Widevine is ON in brave://settings/extensions I will never be prompted to allow the installation and to run Widevine in any website that requires it. This is true even if it is the first time that I try to access to a website that requires Widevine on that profile.