When Lightning introduced by Salesforce many years ago, it still depends on the "Classic" page layout. There has been a rumour for quite some time in the Salesforce community that we will be able to add fields directly in the Lightning app builder without the need for Classic page layout.
This finally comes true with Dynamic Forms. In the Winter '21 release, Dynamic Forms become Generally Available, but only for custom objects. As per, DreamTX 2020, the Dynamic Forms will be available in Summer '21 release, including for mobile support.
How to activate Dynamic Forms?
Dynamic Form needs to be enabled on each Lightning Record Page, so this means that you do not need to enable dynamic form for the whole org at once, you can enable it by the app, profile, or record type. So, you can pilot a profile with dynamic form, while all others still use Classic page layout within the same object.
Enabling Dynamic Forms