That first video had cracking up lol.
It’s hard to differentiate for most people unless it’s a very noticeable/iconic part. You get better at it once you become a player (not just bass but guitar as well) and your ears just kinda get better at listening and dissecting all of the various instruments and what they’re doing. Trying to learn the bass parts from a song by ear helps, but obviously, you wouldn’t do that yet…
The best way is probably look up bass covers on YouTube for some of your favourite songs (from good players) so you can see all the bass is doing and since you know the song already, it’ll be easier to relate it.
It may also be worthwhile to look at concert footage of your favourite bands, isolate the bass player, and try to pay attention to their hands and listen carefully (with head phones) and try to follow along, and hear what they’re playing.
I’m not into a lot of metal tbh, but some bassists EQ/play in ways that make them more audible in the mix. To me these are Iron Maiden (Steve Harris), Tool (any bassist really), Pantera, and maybe Black Sabbath, though I understand they aren’t thrash. Ghost BC usually has good, prominent bassists as well, but it’s certainly not very heavy… I think “From the Pinnacle to the Pit” is their heaviest song, and that intro (bass) is awesome. Cannibal Corpse also has a great bassist (though I’m not their biggest fan).
Also, if they’re just chugging along to the guitars, you won’t really be able to identify it, as it will just give a deeper foundation to the guitars sound.
A lot of thrash bassists also overdrive their tone so much it would sound thin and get kinda lost in the mix. Also, in a lot of new metal (from what I’ve seen), some guitarists are downtuning their guitars to oblivion, getting right into the bassist’s range, taking up their space too, which makes it harder to differentiate as well.
Finally, some bass players in metal overplay. The bass is harder to hear naturally (and in my opinion, is meant to be more of a rhythmic tool that gives a deeper foundation for the melodic instruments). When a bass player is just playing fast 16th note runs, it gets buried as well. The bass players, to me, that stand out, are the ones that emphasis the rhythm. In bass especially, what you don’t play, is sometimes more important than what you do. I love rhythm, and that’s why I play bass.