Unable to read sub folders inside document library with REST

I have a document library with files and folders. I am using the below code to try to get the list of files inside a specific folder that sits in this document library using the SP REST API.

I am using console.log to try to investigate what I get back but I don’t understand how to find the file names.

REST code:

jQuery.ajax({
            url: "https://.../org/eu/EU3/EU34/_api/web/lists/GetByTitle('Goverance Library')/items",
            type: "GET",
            headers: {"Accept": "application/json;odata=verbose","Content-Type": "application/json;odata=verbose"},
            success: function(data) {

                console.log(data.d.results);

                for(let i = 0; i < data.d.results.length; i++) {
                    console.log(Object.keys(data.d.results[i]));
                }

            },
            error: function(data) {
                console.log("Error occurred trying to get data");
            }
        });

One of the folders that is inside the library is titled "SOP" but if I try to include it in the code I get a 404 error. The following returns 404.

url: "https://.../org/eu/EU3/EU34/_api/web/lists/GetByTitle('Goverance Library/SOP')/items"

The following is a couple of snippets from what is returned from the console.log.

One of the results from logging data.d.results:

(100) [{…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}]
0:
AttachmentFiles: {__deferred: {…}}
AuthorId: 7797
CheckoutUserId: null
ContentType: {__deferred: {…}}
ContentTypeId: "0x012000C3ABD68E61A00245B856979F3A10F851"
Created: "2020-07-10T08:31:06Z"
EditorId: 7797
FieldValuesAsHtml: {__deferred: {…}}
FieldValuesAsText: {__deferred: {…}}
FieldValuesForEdit: {__deferred: {…}}
File: {__deferred: {…}}
FileSystemObjectType: 1
FirstUniqueAncestorSecurableObject: {__deferred: {…}}
Folder: {__deferred: {…}}
GUID: "b89a33dd-7b7b-4662-8585-c5283a735676"
ID: 6
Id: 6
Modified: "2020-07-10T08:31:07Z"
OData__CopySource: null
OData__UIVersionString: "1.0"
ParentList: {__deferred: {…}}
RoleAssignments: {__deferred: {…}}
Title: null
__metadata: {id: "Web/Lists(guid'63f6ca95-8834-4aa2-aee2-db93d00a4037')/Items(6)", uri: "https://.../org/eu/EU3/EU34/_api/Web/Lis…d'63f6ca95-8834-4aa2-aee2-db93d00a4037')/Items(6)", etag: ""3"", type: "SP.Data.Goverance_x0020_LibraryItem"}
__proto__: Object

One of the results from logging Object.keys(data.d.results[i])

(24) ["__metadata", "FirstUniqueAncestorSecurableObject", "RoleAssignments", "AttachmentFiles", "ContentType", "FieldValuesAsHtml", "FieldValuesAsText", "FieldValuesForEdit", "File", "Folder", "ParentList", "FileSystemObjectType", "Id", "ContentTypeId", "Title", "ID", "Created", "AuthorId", "Modified", "EditorId", "OData__CopySource", "CheckoutUserId", "OData__UIVersionString", "GUID"]

How can get the list of files inside the folder using REST?

https://server/_api/web/GetFolderByServerRelativeUrl('/Goverance Library/SOP')?$expand=Folders,Files

https://server/_api/web/GetFolderByServerRelativeUrl('/Goverance Library/SOP')?$expand=Files