I have created a subsite https://mycompany.sharepoint.com/sensitive that I want to restrict to a small number of people within my organization. I have created a group, added these staff and gave the group permissions to view and read this site.
The members of the "Team Site Owners" group have access to the subsite also by default.
How can I prevent access for this particular subsite for members of the "Team Site Owners" group?
you can stop inheriting permission on your Subsite and delete the "team site owners" group from the Subsite permissions. Please attention, that this users not in the SiteCollection administrators, otherwise they will still have access to the Subsite.
P.s.: When it's possible, use Sites instead of Subsites...