Microsoft
MicrosoftMortenTherkildsen - Microsoft-provided site templates can be applied to subsites. If you are applying a custom site template, note that it needs to be set up for the site types allowed through subsite creation. More information about the -WebTemplate value here: Get started creating SharePoint site designs and site scripts | Microsoft Docs We recommend leveraging hubs and not subsites moving forward.
Mirosław_IECM - The new site template experience currently cannot be disabled.
rumispoint1 - Customer site templates are what we are calling site designs moving forward. Yes, there is still a limit of 100 customer site templates - how many would you expect to be supported?
You can build a customer site template that includes libraries, lists, schema, navigation, and branding - pages and webparts are not supported at this time. There's currently no UI to create a template from an existing site, but you can refer to the following documentation to help you get started with creating site script syntax from an existing SharePoint site:
Get-SPOSiteScriptFromWeb (SharePointOnlinePowerShell) | Microsoft Docs