A few questions about blockchain security

I'm pretty new to blockchain technology in general, so I'm sorry if these are some really noob questions. They're also not really about ethereum but blockchain in general, hope that's allowed.

Firstly, how does a public blockchain gather a big enough userbase for its systems to function properly? If it's simply released with only 2-3 nodes in the beginning, couldn't an attacker just very cheaply compromise it? Also, what about blockchains that are intended to be used by a niche market - is the only option there to have it permissioned? One solution would be to simply build upon another blockchain (Ethereum, Polkadot, etc), but how did THOSE get their users?

Second, I find all the information out there about the different types of attacks really confusing. Say a "51 / 33 / 66%" attack happens - how does the blockchain recover from this? The blocks added are immutable, so are you supposed to just let it die after the attack happens? Furthermore, how does a blockchain recover from a network split, which only requires 34% of nodes to be malicious?