How to understand Spring profiles?

Question

Java

Hello!
Quite a strange situation with profiles in Spring Boot. Checked in versions 2.0.6 and 2.1.0
It is possible that I misunderstood the principle of how profiles work... I built
a project on Spring Boot ( https://start.spring.io/) via intelllij IDEA Ultimate Edition + using Gradle.
Created 3 .properties files

application.properties
application-dev.properties
application-production.properties

In the main application.properties file I wrote:
spring.profiles.active=dev
Accordingly, I added
server.port=7777 to the application-dev.properties file
1) I start the project and it ignores my configuration and starts on 8080.
2) I tried to register an active profile through the IDE. Edit Configuration -> Active Profiles -> dev (in Intellij there is no need to specify -DSpring...) I
run the project and again it starts at 8080 instead of 7777.
Only if I write the configuration in the main file, it will work.
3) Tried in Application.class via IDE "change profiles" and selected development, same result.
4) I tried to create a separate configuration file for the database and added @Profile("dev") and @Profile("production") to different methods.
At the same time, both methods are worked out ...
Where am I wrong? I watched a bunch of videos and surfed different sites.
PS I did the same manipulations with the yaml file, and also tried the project through the maven.

Answer 1

Thanks to the help, I finally figured out the problem and it was terribly banal)))
In general, when creating a properties file, you must manually add .properties, if you select the properties file type in the IDE, it will simply create a file (not in the properties format.)
Solution link:
Why does Spring ignore profile file properties configurations?
Screen:

Answer 2

Where are these files? They must be either in the class path or in the working directory (for IDEA, this is the default project root).