逗點後為何可用(that)引導形容子句呢?

Question

It is the interaction between people, rather than the events that occur in their lives, (that is) the main focus of social psychology.

1.我不太清楚的是,that is後應是個形容詞子句,為何逗點後可以用(that)呢?

2.that is the main focus of social psychology是修飾(interaction)或是(people)呢?

3.麻煩試譯,謝謝

Answer 1

It is the interaction between people, 【rather than the events that occur in their lives】, that is the main focus of social psychology.

這句話應該這麼看
先把片語的部份遮起來

譯為：
這是人與人之間的互動，這也正是社會心理學關注的焦點所在

這裡的"that"指得是the interaction between people

加入片語的話，譯為：
這是人與人之間的互動，而不是生命中偶然出現的活動，人與人之間的互動正是社會心理學關注的焦點所在

Answer 2

該句更明確一點說應該是強調句型的分裂句：
it is ..... that .....