The source code for this component can be found here:
https://github.com/SteveSandersonMS/BlazorInputFile
I studied the code and found that this component is built using the standard Blazor input type.
<input type=file>
Steve shows a way to override the default functionality and style of the button using CSS.
Here is an example I created based on what I found:
<style>
.file-input-zone {
display: flex;
align-items: center;
justify-content: center;
background-color: blue;
color: white;
cursor: pointer;
position: relative;
width: 120px;
height: 30px;
}
.file-input-zone:hover {
background-color: lightblue;
}
.file-input-zone input[type=file] {
position: absolute;
width: 100%;
height: 100%;
opacity: 0;
cursor: pointer;
}
</style>
<div class="file-input-zone">
<InputFile />
Get me a file!
</div>
It will give you a button that looks like this:

You might pick up more tips by studying his source code further.