Raid1 on boot howto

I would like to know how to install properly with Raid1? I tried installing with btrfs and raid1. Then I wanted to simulate what happened if I removed one drive would fail so I removed one drive and then it just did not boot anymore and I ended up in some kind of emergency shell. I’d expected that raid1 would make it run even if one drive failed.

So how to set it up properly and how to recover if one drive would fail.

thank you.