Technically, what it means is that you need to sell the app and an offer for the source code. It might be semantics, but it's not really accurate to say you have to "give the source away" -- you could also say that you sell access to the source and give away the binary as an added service...

> Technically, what it means is that you need to sell the app and an offer for the source code.

Only for the library (and any modifications to it), not the app itself.

If the developer makes modifications to the actual source code of Qt itself he/she has to release those modifications[0]. He/she does not have to release the rest of the source code of the app.

[0] Technically, the Qt source must be provided whether or not any modifications were made, though this is easily fulfilled.

For iOS there never was a free option, one has to buy the commercial version. For Android it should work just the same as before as far as I can determine (but I might be wrong).