Hi there!
Thanks for contacting us.
Your mobile menu is right there. Please click on the 3 bars icon as indicated in the screenshot, then you will find the menu in mobile: https://prnt.sc/LZd4zdR0LPod
It's called offcanvas or hamburger menu, designed for mobile devices intensionally.
If you want to change your mobile header space then you can use the following code in your custom CSS:
@media (max-width: 575px){
#sp-header {
height: 60px;
}
}
Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
Best Regards