You can append span which you have created inside each loop under th-inner so that it will appear side by side of the header .Also , you can hide filter-control div this contains the filter which you need to use either input or select.So , whenever ? is clicked just get reference of this div to show/hide same.
Demo Code :
$(function() {
$('#table').bootstrapTable()
$(".filter-control").hide() //hide filter control
$('#table thead th').each(function() {
s = $('<span style="position: absolute;margin-left:10px">?</span>')
s.click(function(e) {
e.stopPropagation();
//get closest filter control div hide/show same
$(this).closest("th").find(".filter-control").toggle();
});
//append span inside th-inner class
$(this).find(".th-inner").append(s)
})
})<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css" integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/" crossorigin="anonymous">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-table/1.8.1/bootstrap-table.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js" integrity="sha512-bLT0Qm9VnAYZDflyKcBaQ2gg0hSYNQrJ8RilYldYQ1FxQYoCLtUjuuRuZo+fjqhx/qtq/1itJ0C2ejDxltZVFg==" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
<script src="https://unpkg.com/[email protected]/dist/bootstrap-table.min.js"></script>
<script src="https://rawgit.com/wenzhixin/bootstrap-table/master/dist/extensions/filter-control/bootstrap-table-filter-control.js"></script>
<table id="table" data-filter-control="true" data-show-search-clear-button="true">
<thead>
<tr>
<th data-field="id">ID</th>
<th data-field="name" data-filter-control="input">Item Name</th>
<th data-field="price" data-filter-control="select">Item Price</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>Item 1</td>
<td>$1</td>
</tr>
<tr>
<td>2</td>
<td>Item 2</td>
<td>$2</td>
</tr>
</tbody>
</table>CLICK HERE to find out more related problems solutions.