Custom master pages are currently not supported for "modern" pages.
You can find supported/unsupported customizations in below documentations:
Customizing "modern" site pages
Customizing "modern" team sites
Branding your SharePoint site
There is no SharePoint out of the box option to hide office 365 ribbon at the top.
Once you delete a group, it will be retained for 30 days by default.
This 30-day period is considered a soft-delete because you can still restore the group.
After 30 days, the group and its associated contents are permanently deleted and cannot be restored.
Restore a deleted Microsoft 365 group
Restore a deleted Microsoft 365 group ...
It is not possible to show the dialog/pop up on button click using out of the box functionality.
If you are using text web part then you can create a hyperlink which will open a video/link in new browser tab Or you can use button web part to show button & open link in new tab (This is what you can achieve using out of the box web parts & not dialog/...