This template uses custom code for two things:
The custom code is located in the site settings -> custom code, both in the "Head code" and "Footer code" sections.
The Head code CSS is only setting a few elements to "opacity: 0" in order for them to stay hidden when the page loads.
The Body code javascript is initializing the Lenis smooth scrolling library and is page load animating all text elements with a class of Gsap Text Load, Title and Subtitle.
This template uses a modal for the RSVP form. This modal can be found and customized by editing the "RSVP Modal" component. You'll find the component through the component navigator on the right hand side of the Webflow designer.