According to a Twitter response from 2016, Eurotunnel at least was the usual way for FlixBus.
The review here indicates both ferry and Eurotunnel train are possible for FlixBus.
Whether a particular bus takes the ferry or Eurotunnel can be checked (at least sometimes) by looking at the schedule, but the mode of crossing could still change depending on ...
Barcelone-Bucharest is 2700+km and takes between 48 and 51 hours on schedule.
If counting by linear distance between cities, London-Bucharest is 2090 km (the bus takes about 46 hours if on schedule) while Barcelona-Bucharest is 1970 km.
Don't sweat the fine print too much.
In practice, you can take any train or bus within the travel zone and the 120 minute validity period - provided you don't go back to where you came from. The is no limit on the number of changes or stopovers.
The ticket inspectors do not know your destination, nor can they tell how often you have changed (or plan to ...
In practical application the question is, at the time you ticket is checked are you travelling clearly towards the station where your ticket was originally stamped. If the answer is yes, then it looks like you are doing some kind of return trip which is not allowed. In all other situations you are ok. So doing a bunch of unnecessary changes is fine as long ...
The validity of the single tickets is, as described in VBB's terms and conditions:
Diese Einzelfahrausweise berechtigen zu einer Fahrt mit beliebigem
Umsteigen in Richtung auf das Fahrtziel über den reiseüblichen oder
durch die Fahrplanlage bedingten Weg.
The single fare tickets allow a trip with any number of changes in