How do authors ensure that readers will connect with characters through reactions or interaction with environment?

One major way that authors can help to promote a connection between readers and characters is through their choice of point of view. Certain points of view permit readers access to a character's thoughts and feelings about the events that take place in the text, and knowing more about characters' reactions can certainly lessen the perceived distance between reader and character. For example, all first-person points of view include narrators who speak from personal experience of the story's events and who often participate in the story. First-person subjective narrators tell the story as events unfold, in the present tense, and first-person objective narrators tell the story after the events have taken place, in the past tense. Because a first-person subjective narrator has not had time or opportunity to reflect on events—because they are narrating these events as they occur (you might think of them as happening in "real time")—their narratives tend to be more exciting and raw; we cannot know what comes next because they do not. The revelation of their honest, visceral reactions may help to establish more of a connection with readers since we go through whatever they go through as they go through it; this closeness might inspire more sympathy than we would feel for someone who can more calmly detail events that have taken place in the past.
Another option available to authors is the third-person limited omniscient point of view. This perspective means that the narrator is not a participant in the story itself but can tell us the thoughts and feelings of just one character (this is the "limited" part of the description). This point of view contrasts with the third-person omniscient perspective, in which the narrator can tell us the thoughts and feelings of all the characters. These narratives can have a slower pace than the limited-perspective narratives because there is simply more information to tell the reader, but they also keep the reader at the same emotional and figurative distance from all characters. By choosing to use the third-person limited omniscient perspective, in contrast, the author decreases the distance between the reader and that one character whose thoughts and feelings the narrator knows, allowing readers a greater opportunity to establish an emotional connection with them. The narrator can tell us not only how that character appears to engage with their environment and the characters around them but also how they react privately, what they think and feel about those interactions—information that we might not get if the author chooses a third-person objective perspective (where the narrator cannot relate the thoughts or feelings of any of the characters).
Ultimately, point of view may be the most powerful way that authors create a sense of connection between readers and characters, as it allows us to understand and relate to a particular character or characters in the story.