I don't understand how the schema works

[SebastianStehle]

I created a custom extension that works on links and shows them in a special way. The extension looks like this:

@extension<ContentLinkExtensionOptions>({
    defaultOptions: {} as never
})
export class ContentLinkExtension extends NodeExtension<ContentLinkExtensionOptions> {
    public get name(): string {
        return 'contentLink' as const;
    }

    constructor(options: ContentLinkExtensionOptions) {
        super({ ...options, disableExtraAttributes: true });
    }
    
    public createTags() {
        return [ExtensionTag.Block, ExtensionTag.TextBlock, ExtensionTag.FormattingNode];
    }

    public createNodeSpec(): NodeExtensionSpec {
        return {
            attrs: {
                contentId: { default: '' },
                contentTitle: { default: '' },
                schemaName: { default: '' },
            },
            toDOM: node => {
                const href = `${this.options.baseUrl}/api/content/${this.options.appName}/${node.attrs.schemaName}/${node.attrs.contentId}`;

                return ['a', { href }, node.attrs.contentTitle];
            },
            parseDOM: [
                {
                    tag: 'a[href]',
                    getAttrs: (dom) => {                        
                        const href = (dom as HTMLAnchorElement).getAttribute('href');
                        const content = getContentId(href, this.options.baseUrl, this.options.appName);
                        
                        if (!content) {
                            return false;
                        }

                        return {
                            contentId: content.id,
                            contentTitle: (dom as HTMLElement).innerText,
                            schemaName: content.schemaName,
                        };
                    },
                    priority: 100000,
                },
            ],
        };
    }

    public ReactComponent = ContentLinkRenderView;
    
    @command({})
    public addContent(content: Content, selection?: PrimitiveSelection): CommandFunction {
        return this.store.commands.insertNode.original(this.type, {
            attrs: {
                contentId: content.id,
                contentTitle: content.title,
                schemaName: content.schemaName,
            },
            selection
        });
    }
}

It works as expected when adding nodes and so on but not when loading text.

If I understand it properly prosemirror has a model that defines which elements can be added where, so for example a paragraph can have a text and image.

So I had a look to the heading-extension and copied the text from there and what I noticed:

1. A heading can be part of a paragraph and part of the root. So the following examples are valid:
   • <p><h1>Headline</h1></p>
   • <h1>Headline</h1>
2. My contentLink can not part of a paragraph, so only this is valid:
   • <a href="MATCHING_URL">Text</a>

In markdown everything is part of a paragraph, because it is added when the markdown is converted to html.