I wanted to make an IoT device using RaspberryPi, and when I searched for a smartphone that could be operated, I found that it was easy to connect and operate using an app called Blynk, so I tried it.
You can connect the Raspberry Pi to your smartphone via the Blynk server, and you will be able to operate the scripts on the Raspberry Pi using the Blynk app.
The whole picture looks like this. (Source: http://docs.blynk.cc/)
Preparations are required on the smartphone side and the Raspberry Pi side respectively.
[email protected]:~ $ git clone https://github.com/blynkkk/blynk-library.git
[email protected]:~ $ cd blynk-library/linux [email protected]:~/blynk-library/linux $ make clean all target=raspberry
[email protected]:~/blynk-library/linux $ sudo ./blynk --token="Token arrived in the email"  ___ __ __ / _ )/ /_ _____ / /__ / _ / / // / _ \/ '_/ /____/_/\_, /_//_/_/\_\ /___/ v0.6.1 on Linux  Connecting to blynk-cloud.com:80  Ready (ping: 192ms). ---
Blynk is now up and the connection between your smartphone and Raspberry Pi is complete via the server.
With Blynk, it was possible to connect a smartphone to a Raspberry Pi in a short time. After that, you can create what you want to do in the project and create an executable program to execute it via the Blynk server.