Looking for help with the WordPress Plugin Boilerplate

I'm building a WordPress plugin that can display your Discogs collection. I am building it with the WordPress Plugin Boilerplate which is object oriented.

I was able to setup an admin settings page which stores a Discogs API consumer key, consumer secret, and their username. All that works just fine, but from here is where I am a bit lost about the placement of functions. I understand that the WP Plugin Boilerplate has separate classes for code that runs in the admin only, on the front-end only, or on both.

I have functions like the ones below and I really have no idea where to place them and then how to use them.

Example functions:

<?php
function wp_discogs_get_collection() {
    // function that retrieves a user's collection
}

function wp_discogs_get_release_artwork($release_id) {
    // function that retrieves the URL for album art for a release
}

function wp_discogs_get_profile() {
    // function that gets a users profile information
}

Pretty much all of these functions will run on both the back-end and the front-end. Do I want to create a new class for a "collection" and have these as methods on the collection? Or do I place these all in the main class that is in the /includes folder?

Thanks!