uBlock Origin's advanced mode does pretty much the same thing as uMatrix.

Which engine? This is new to me. I thought ABP is so slow because the code is crappy (thats also what i see when i look at the code)

Ditto except uMatrix

What exactly does it do?

uMatrix provides a tabular view sorted by host featuring toggleable category columns e.g. Dis/Allow iframe, script etc. It's granular client side resource whitelisting.

uBlock Origin does that too, just need to turn on Advanced Mode via options.

uMatrix offers more granularity. You can choose exactly what each third-party site can do in terms of cookies, CSS, images, plugins, javascript, XHR(!), frames and media (audio, video, pdf, etc).

After years of using uMatrix (formerly HTTP Switchboard), many sites "just works" wrt Youtube, Vimeo and similar even without first-party javascript enabled.

I've considered sharing parts of my global ruleset so others can just copy-paste the sections/sites they want to whitelist without having to discover what's required themselves.