Structuring a simple internal web application tool with OAuth

I'd still consider myself a newbie web dev so excuse me (and please correct me) if I say anything that seems absurd in this post.

I'd like to create a internal facing tool for a small team of ten that can automatically compose emails periodically in each team member's account. According to my research, this can be done if they authorize once with Google's OAuth 2.0 and I just keep the access token and use it whenever I need to compose emails. Every day the application needs to have some logic to figure out what emails to send that day, and (1) compose those emails (2) send that information to a slackbot.

My confusion is around how the different parts of the application fit together at a high level and what parts of a standard web application (front end view, API, database) are necessary for such a simple application. What is the simplest possible way to do this? My first thought was just to have a server side script that does all this, but then I needed to have some way to be initially authorized by each person. It seemed like I needed some routes that people could visit that would initiate the whole OAuth back-and-forth and act as the redirect_uri. I think I'd have to integrate a whole other framework to do that, like Flask. Is this overkill / is there any other way to do simple authorization without having to do all this just for a few routes? I'm having trouble visualizing what this app would look like and how the parts would fit together/communicate.

If anyone can just clarify how the different parts of the application would be implemented/structured, that would be really helpful.

Thanks in advance!