You can fork from source code and create your altcoin. I don't call it as build an altcoin as the work is simply for coder. Scammers use source code, fork it and create scam altcoin without any product and value.

Applications don't have source code as I know. In my approach, I daren't accept and install application of unknown altcoins. My devices might be compromised by malwares, trojans, many threats.