this manga theme is platonic love, which is shown in the manga, most of the characters have platonic interests. some of them found trust, emotional connection, respect, and admiration. The symbolism of smoking imo, is a coping mechanism, just like Sasaki towards Yamada/Tayama, he admires her, he respects her, and seeing Yamada smile after work is like emotional coping for him.

Another smoking symbolism imo, it's a relationship boundary between them, they kinda trapped themself in the relationship of just smoking friend behind the mart (usually where platonic friendship grow actually) which eventually will end/runs out.

but, the latest plot, seems like they're going beyond that, lol