sharepoint enterprise – Adding a user to different groups of permissions in the same site collection?

This is for SharePoint 2019 Enterprise Site.

What I need to build is a site in which each site and subsite has its own unique access authorization groups. Creating the site and the subsite and telling subsites not to inherit was the easiest part. the hardest part is how to add or update a user's permissions on multiple subsites in the same site collection.

Owners group
Group of members
Group of visitors

Subsite A
A Owners Group subsite
Group of members of sub-site A
Group of visitors to subsite A

Sub-site A-1
Owners group of subsite A-1
Group of members of subsite A-1
Visitor group of subsite A-1

Sub-site A-2
Owners group of subsite A-2
Group of members of subsite A-2
Visitor group of subsite A-2

So I want to add a user with read-only access to the main site /, sub-site A and sub-site A-1, but not sub-site A-2, but the only way up here seems to go manually to each subsite of the collection, click on the group of visitors and the addition of the user. What do I miss?