To add to this: there are staking masternodes and proof of work masternodes. Staking masternodes is like you wrote: you just have to keep a certain amount of tokens in a wallet. For a proof of work masternode, you need to keep the wallet online. The blockchain depends in that case on those wallets to keep the network up. I personally only have a staking masternode. I haven't bothered setting up a system that needs to be online all the time.