It's a tired cliche that when planning your season, you start by working on defending. I'm convinced that if you want to have a possession-based team, that's what you should be working on right out of the gate. For example, Pep Guardiola's first training session with Manchester City seems to feature a heavy focus on building from the goalkeepers.
It makes sense when you think about it. If you could choose one thing for your team to do well, wouldn't it be retaining the ball? If you can do do that, your attacking and defending will both be easier. If you begin your season by focusing on defending, then that's what your team will be most proficient at in games. But is that really what you want your team's identity to be? Ideally, I only want to defend in short, intense bursts. Therefore, that's how I structure my defensive sessions.
On the subject of season planning... I want my team to have the majority of possession in games. Therefore I structure my season around possession exercises. I spend less time on defensive topics because I don't want to spend much time defending in games. That doesn't mean it's neglected, but it's important to prioritize training based on what you want to see in games.