These are well done and succinct videos to understand the 51% attack on a proof of work blockchain like Bitcoin and Monero. And we’ve seen an attempted 51% attack on Monero, but more than likely they never got to 51%.
And a key takeaway for Bitcoin users is that miner centralization is a problem. And he goes over some of the threats involved. Myself, I like the nuclear option of nuking all the ASIC miners as that would be disruptive, and a serious threat to a lot of investment in these ASIC miners.