For anybody else who had as much trouble finding global search as I did and who stumbles across this thread looking for an answer, here’s how you get to it:
Tap the button in the top left to open the navigation drawer (I don’t know the official name for this and the button is only labeled ‘expand’)
By default you’ll now be looking at ‘Files’
Tap the word ‘Files’ or the icon to its right of two arrows pointing in opposite directions - now you’ll see (with no plugins installed, at least) a list that contains:
Files
Search
Tags
Starred
Tap on ‘Search’
I don’t know if this is the trouble the person who posted this question asked was having, but before discovering this, I stumbled across the graph view filter pane first - it has a text field confusingly labeled ‘Search files…’ - and was confused when it didn’t search my files.