From the same place where value of a roulette game comes from: from belief, hope, faith and opportunistic gambling of people that have enough coins to place them on many different numbers hoping to get some cash before the house. When you bet some coins on a number in roulette, you get no ownership and you'll get no dividends from the casino - but you may end up with some cash when the wheel stops.
Bubble is already here. And it will blow. But some players, that have a substance, will survive or rise from ashes. And they will be very valuable.
The tech will also survive and evolve.
This happens all the time - dotcom bubble was the same, but there are more examples: we all know now that alchemy was a total scam, with lot of shady players milking naive people, but eventually from its residues the real chemistry was born. Hopefully we will learn one day how to jump over this kind of inital scams.
Actually, very often they don't... Porsche shares, if you wish, are the representation of the public perception of value of their cars, service network, brand, cash in the bank, etc. They are the quantifiable proxy of those values. Cryptocurrencies do not proxy anything. So, the only way to measure their value is by "what people are willing to pay". And that doesn't have to be bad, a-priroi. Its just different thing. So, the cap of crypto _is_ nr of coins time price per coin, where ppc changes heavilly over time. So the "market cap" of crypto is not constant - it changes very rapidly over time. <crypto_name_here> could have a cap in billions today and zero next Thursday. Hence, there isn't much sense in trying to define/measure it.
Check out hyperledger.org - Fabric (a blockchain implementation) and Composer (tool to build apps/network on top of Fabric). Docs are pretty good, and there are no built-in currencies, coins, tradings and similar crap. And it is relatively easy to have it up and running locally and to deploy custom code on it (it is Go)