When does a web app need a mobile-first experience?

I’ve been thinking about this while working with an interview-practice platform that already runs as a web application.

The core experience works well on desktop, but the way people use the product on mobile is quite different.
Interview practice can involve voice interaction, microphone access, short practice sessions, progress tracking, and returning to the platform regularly.

That makes me wonder -
whether simply making the website responsive is enough for this kind of product.

At what point does it make sense to give users a dedicated mobile experience rather than continuing to optimize the browser version?

I’m particularly interested in the trade-off between improving an existing mobile website versus building a dedicated app, especially when the product involves frequent voice/video interaction.

For builders who have faced a similar decision, what made you choose one direction over the other?