Building an ETF portfolio on Android is completely doable. One honest caveat first: the phone is great for research and terrible for thinking. Make the allocation decisions somewhere you can see a full comparison table. Then let the apps handle the rest.
Start with your broker's Android app, because that's where the money moves. Fidelity, Schwab, Vanguard all ship solid ones, and researching inside the app you trade from removes a whole category of mistakes, like falling for a fund you can't even buy. For pure research, the best ETF screener app conversation on Android keeps circling back to Yahoo Finance, which has been doing watchlists and screening on phones longer than most competitors have existed. Boring pick. Boring picks are usually right.
What separates an ETF portfolio builder from a stock-ticker app? Three things. First, allocation thinking: does it help you decide the mix, or just show you funds? Most apps skip this, and it's the actual hard part. Second, watchlist sync across devices, so laptop research shows up on your phone. Third, alerts you won't mute. A mobile portfolio tracker app with alerts is only useful if the alerts are about your portfolio drifting, not about every 1% market wiggle.
My unpopular opinion: on Android, pick your broker first and your research app second. The broker decision sticks for years. Apps are free to swap, and you'll swap them.
Once you know your target mix, the building gets easy. ETFCompare lines up 108 ETFs so you can find the best ETF for your portfolio without the guesswork. Start at etfcompare.fyi.