How does matchmaking work? Insane losing all of a sudden

That link is pretty out of date. See How Competitive Matchmaking and Rating Works (Season 18) for an updated version.

Competitive MMR is separate for each role, and separate from other modes.

No.

SR has no effect on matchmaking. In theory, if your MMR gets significantly separated from your SR, you could see effects where your team SR is always higher/lower than the enemy. I don’t think this happens anymore, though, so unless you wrote down all of your data (your SR, your team SR, enemy SR) it is likely just random fluctuations. Even if it did happen, SR doesn’t matter in matchmaking.

Streaks are just a consequence of mathematics. Tilt can exaggerate them, but not being tilted can’t eliminate them.

Win probability changes slowly with rank because there are so many random factors in each individual match. Unfortunately, it follows from this that frequent and long streaks will occur, and a player’s rank will oscillate widely. Essentially, a player will tend to bounce between the range of where he is nearly guaranteed to win and where he is nearly guaranteed to lose. This problem can be analyzed in depth, mathematically: See Overwatch Forums and https://www.reddit.com/r/OverwatchUniversity/comments/aatezy/why_match_quality_is_frequently_poor/

Finally, you will see some pretty crazy ideas on the forums. They are a trap. If you play better than your rank, and play enough games to overcome random factors, you will rank up.