Question about ETH2.0 and DeFi apps based upon POW ETH.

Maybe this isn’t the smartest question, but I am having some difficulty understanding what the implications to the DeFi (those operating on ETH) world will be when ETH upgrades to 2.0 (POS). Will the existing dapps port over seamlessly? Do they not port over? Or do they have to port over to the new POS chain? Can they leverage ETC, if they decide not to? Are there any technical challenges they need to worry about or is the expectation that ETH will manage most of the work required for this transition ? I’ve been doing some reading and just want general feedback on those who have thought about this.

Any guidance here would be highly appreciated.