I don't remember Union Berlin joining the Champions League before. This must be their first time I assume. Therefore they are going to be an inexperienced team in the tournament. However I'm pretty sure they will care about it much more than the Europa League. Because they can increase their income significantly by getting positive results as much as possbile.
They need to get lucky with the draw as well though. If they have a really tough group in which all teams look challenging then Union Berlin might not do much. Even if they drop down to the Europa League at the end it would be great for them anyway.
Union Berlin needs to improve far enough if they want to perform well and be able to compete in the Champions League next season, although they get an easy draw and can qualify for the group phase the knockout stage will certainly be different and difficult to survive when facing other teams who have good experience in this tournament, maybe Union Berlin will focus more on domestic competitions to be able to finish in the top four again while continuing to develop their team more and more It's even better.
The Europa League may be different but wherever they will compete they have to prepare well and with the same good core and reserve players so there will be no obstacles when rotating skuad because they have to play many games due to competing in several tournaments.
Union Berlin was promote on season 2018 and since that they can managed their position and playing at european competitions and next season Union Berlin will be very busy because playing at champion league but they have to upgrade their squad because many strong teams were available at champion league so if Union Berlin want advance to next round then they should be buy some of good quality players this summer
I think one of successful key from Union Berlin to finish at 4th place is they can utilized home matches with very well and last season Union Berlin became an the team who very strong at their home and i have seen their statistics that during play for home matches Union Berlin was unbeatable they even can able to beat several strong teams such as Leipzig, Dortmund and draw from Bayern Munich so i think this is fantastic records because Union Berlin is the team with lack of star players but at Europa League their journey have to stopped at round of 16 because lose from Union Saint-Gilloise