(New to SharePoint here!) We're currently setting up a Site Collection with different subsites utilizing the Modern Pages style. In the main parent site we have a list of all the subsites so people can go to any subsite from there, but once they get to their subsites, there's no link going back to the parent site.
I just found out that SharePoint's Modern Pages do not support Custom Master Pages and/or JavaScript injection so we couldn't insert a hyperlink on the left-hand menu to let the users go back to the parent site. Are there any other ways to go about it?
If I understand the problem correctly, You would like to have a quick link/top link from sub site to navigate back to parent site. If yes, Irrespective of classic or modern SharePoint site, you could navigate to site settings --> setup Quick launch and/or Top link bar, and configure your left / top navigation based on your need.
Quick link
On your other questions on customizing / javascript injection , on modern pages/ sites, You could potentially do it through SPFx extension.
Let me know if you need more details.