You can create mobile workspaces and open them with the D365FO mobile app. These are basically very simple D365FO forms.
Then there are separate Microsoft made apps for some scenarios, such as warehousing and timesheet entry.
In addition to that, you can do PowerApps that talk with D365FO data entities.
And, you can integrate your proprietary mobile apps with D365FO services.
So there are many tools, and a lot of information available about all of them.