Content Types via REST API or Graph

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Im trying to create content type and assign custom site columns / fields to it via REST API in SharePoint Online. The HTTP action works partially as all new CT are create with PARENT CT item... I dont know how to pass the Parent info in the call... Could you please help with that? The next step is to assign some fields/site columns to created Content Type dont know how to deal with that... If there is a better way to do it via MS Graph Im more than happy to try it.
Know how to do it with CSOM or PnP PS but would like to achive that with Flow that is why thinking to use REST or GRAPH
Till now I tried to use _api/web/contenttypes and
'__metadata': {'type': 'SP.ContentType'},
'Description': 'TestDesc2',
'Name': 'ContentType New7',
'Group': '_GroupCustom',
'Parent': {'__metadata': { 'type': 'SP.ContentType' },
'Id':{'__metadata': { 'type': 'SP.ContentTypeId' },

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