Yes, its important for tracking that authorized participants are able to both create and redeem the ETFs for the underlying. This is why the Grayscale family does such a god-awful job of tracking the underlying. Check out the premium over time. [1]

I suspect these are more like perpetual futures or CFDs?

[1] https://ycharts.com/companies/GBTC/discount_or_premium_to_na...

Not always. There are synthetic ETFs backed by nothing but swap contracts.

For example, commodities ETFs typically work by throwing cash in to treasuries and purchasing the total return swap contracts between those treasuries and some benchmark of commodity future contracts.

The major difference with these blockchain tokens is that the collateral (the stablecoin) held against the stock benchmark is itself completely synthetic.