Hello all! The CodeWithChris link above is helpful even if it's not directly about SwiftUIm thanks.
For me, the breakthrough was realizing that you must not include the font path in the font item, i.e. "item 0" must be "MyFont.ttf" and not "custom_fonts/MyFont.ttf" -- contrary to what various tutorials have been telling me.
Also worth noting: in XCode 15 with an iOS app under SwiftUI, actually creating an Info.plist file caused build failures that were really hard to debug. Apparently Info.plist isn't the normal way anymore, and you need to add properties in the XCode UI directly for the target.