I installed sitecore horizon module on sitecore 9.3 environment. followed the installation guide provided by sitecore and followed all the steps accordingly. But after installing i could see only default sitecore home item in my horizon UI. My expectation it should pick the existing content tree as per site definition.config. could you please suggest where i am going wrong ?
Horizon does not support multi-site on the 9.3 environment. It does as from Sitecore 10. In that version you can switch sites to see the content tree for the site you desire:
In the 9.3 version Horizon will open with the default site. So you need to get your site as default. I don't have a 9.3 with Horizon to test here, but you could try to move (order) the sites in the sites definition and/or set the
Preview.DefaultSite setting. (https://doc.sitecore.com/developers/93/platform-administration-and-architecture/en/determining-website-context.html) This should help for the other editing environments but I'm not sure if it will also affect Horizon.