I'm not sure about Ripple, but I know IOTA specifically uses each transaction to confirm the transactions around it. So instead of a dedicated miner, each user is essentially a "miner" for the short time they are making a transaction.

You need some way of coming to a consensus about the accepted transaction history. Proof-of-work is one mechanism which works robustly but requires a lot of resources. Proof-of-stake as used in some altcoins is related but less resource hungry. Ripple has a consensus algorithm that isn't completely trustless if I understand correctly. Byteballs (and some other coins) have a "tangle" instead of a chain, which eases the building of a consistent transaction history in the presence of network delays.

Stratis uses a mix of PoW/PoS It started as PoW for mixed PoW/Pos for first few thousand blocks then only PoS

PoS = proof of stake and it's another consensus protocol The concept is that you don't mine (you don't need to waste resources) but you mint, using your coin stake.

I'm not going into details, you can google about PoS, I just note that Stratis is implementing cold staking that improve the PoS minting.

Note that PoS involve anyway a target and someone that find the solution for that target, what it changes is the "how". Consensus is applied at the same way, every node checks for conensus rules for every block it receives, so nothing changes except how and who generate a block.

As far as I understand Ripple, every transaction you make you do the work to send and receive, which is like self serve supply the compute power required.