Signed NDA. But phone interview questions were same as mentioned many times here: flatten array, optimize a function, implement a simple Observable.

Was the architectural have to do with a trie or hash table?

VanillaJS (important!), basic JS data structures including ES6 ones, their usage nuances (what?/when?/why?), at least basic algorithms (array sorting, tree traversal), big-O. Not sure about HTML/CSS; as some noted, that there were also non-JS questions, but my phone interview was focused on algorithms, DS, and big-O.