Bootstrap 4 | Collapse other sections when one is expanded

just put data-parent="#element_parent",

example...

    <a class="btn btn-primary" data-toggle="collapse" href="#colla1" role="button" aria-expanded="false" aria-controls="colla1">
        content 1
    </a>
    <button class="btn btn-primary" type="button" data-toggle="collapse" data-target="#colla2" aria-expanded="false" aria-controls="colla2">
        Content 2
    </button>

<div class="collapse" id="colla1" data-parent="#element_parent">
    <nav
        your content 1 here
    </nav>
</div>

<div class="collapse" id="colla2" data-parent="#element_parent">
    <nav
        your content 2 here
    </nav>
</div>

Make use of data-parent attribute:

<div class="container" id="myGroup">
    <p>
        <a class="btn btn-primary" data-toggle="collapse" href="#collapseExample" role="button" aria-expanded="false" aria-controls="collapseExample">
            content 1
        </a>
        <button class="btn btn-primary" type="button" data-toggle="collapse" data-target="#collapseExample2" aria-expanded="false" aria-controls="collapseExample2">
            Content 2
        </button>
    </p>

    <div class="collapse" id="collapseExample" data-parent="#myGroup">
        <div class="card card-body">
            Content 1 here 
        </div>
    </div>
    <div class="collapse" id="collapseExample2" data-parent="#myGroup">
        <div class="card card-body">
            Content 2 here 
        </div>
    </div>
</div>

I added the id to your container and also added the data-parent to your content sections, referencing this container through the id.