Based on your description, I understand that you want to move a subsite to another site collection in the SharePoint Online.
Here are steps:
1.Allow custom script in the site1 and site2 by using PowerShell.
#Config Parameters
$AdminSiteURL="https://contoso-admin.sharepoint.com"
$SiteURL="https://contoso.sharepoint.com/sites/site1"
#Connect to SharePoint Online Tenant Admin portal
Connect-SPOService -URL $AdminSiteURL
#sharepoint online enable custom scripts powershell - Disable DenyAddAndCustomizePages Flag
Set-SPOSite $SiteURL -DenyAddAndCustomizePages $False
2.Go to Save as template through this link: https://contoso.sharepoint.com/sites/site1/abc/_layouts/15/savetmpl.aspx -> Include content -> Save.
3.Go to https://contoso.sharepoint.com/sites/site1 -> Site settings -> Solutions -> Click the abc subsite wsp file to download it.
4.Go to https://contoso.sharepoint.com/sites/site2 -> Site settings -> Solutions -> Upload solution -> Upload the abc subsite wsp file.
5.Select the newly upload solution -> Activate it.
6.Go to https://contoso.sharepoint.com/sites/site2 -> Site contents ->New subsite -> Select a template -> Custom -> Choose abc.
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