actually not to all System Entities but only to Extendable entities.
if you already have installed system goto System -> Entities.
here you will see a list of all Configurable entities, and some of them marked as “Extendable” – column “IS EXTEND” in datagrid.
Also any entity created in UI or via entity_extend.yml(predefined entity/fields on install) will be extendable too.
So, you can “click” any extendable entity in UI, and you’ll see a list of fields of this entity and a button “Create Field” in top right.
Click it, and follow 2-steps form to create new field.
After that return to fields list.
In top right you will see red button “Update schema”.
New field(s) will not be accessible on views or forms until it is applied to DB.
So, click “Update schema”, confirm and wait, regulary it will take about 5-10 seconds for update, but it depends on your server configuration.
After “Schema update” everything should be fine.
Any way, feel free to ask more questions if something not clear.