$ cd /tmp

$ rm -rf upstream/ myfork/

$  mkdir upstream

$  cd upstream

# == prepare its `master` and `dev/br-1` branches

$  git init

seq 12 | tee file-a
git commit -am 'add file-a'

$  git checkout -b dev/br-1
Branch 'dev/br-1' set up to track local branch 'master' by rebasing.
Switched to a new branch 'dev/br-1'

sed -i 's/^1/7/' file-a

git commit -am 'update file-a'

seq 5 6 | tee -a file-a
git commit -am 'more updates'
seq 12 | tee file-b
git commit -am 'more updates'

 cd /tmp
 git clone upstream myfork
 # == prepare my own changes based on the `dev/br-1` branch into `dev/br-2`

$  git branch -avv
* master                  2f5eaaf [origin/master] add file-a
  remotes/origin/HEAD     -> origin/master
  remotes/origin/dev/br-1 0006c5e more updates
  remotes/origin/master   2f5eaaf add file-a

$  git checkout --track origin/dev/br-1
Branch 'dev/br-1' set up to track remote branch 'dev/br-1' from 'origin' by rebasing.
Switched to a new branch 'dev/br-1'

$ git checkout -b dev/br-2
Branch 'dev/br-2' set up to track local branch 'dev/br-1' by rebasing.
Switched to a new branch 'dev/br-2'

sed -i '/9/{ N; N; s/^.*$/3\n4/; }' file-a

$ cat file-a
7
2
3
4
5
6
7
8
3
4
72
5
6

git commit -am 'update file-a in dev/br-2'

 cd /tmp/upstream
 # advance its `master`

 sed -i 's/^5/55/' file-a
 git commit -am 'update file-a from main'

 # and its`dev/br-1` branches
 git checkout dev/br-1
 git rebase -X theirs master dev/br-1

$ cat file-a
7
2
3
4
55
6
7
8
9
70
71
72
5
6

# Now upstream has advanced, both in its master and its develop branches (via rebase), and I need to pick-up from my own repo to continue with my customization.

 cd /tmp
 mv myfork myfork0
 git clone myfork0 myfork1
 cd myfork1
 git remote -v
 git remote add upstream /tmp/upstream
 git remote -v
 git fetch upstream
 git rebase upstream/master

 git checkout --track origin/dev/br-1

$ git remote -v
origin  /tmp/myfork0 (fetch)
origin  /tmp/myfork0 (push)
upstream        /tmp/upstream (fetch)
upstream        /tmp/upstream (push)

$ git branch -avv
* dev/br-1                  0006c5e [origin/dev/br-1] more updates
  master                    dc45a94 [origin/master: ahead 1] update file-a from main
  remotes/origin/HEAD       -> origin/master
  remotes/origin/dev/br-1   0006c5e more updates
  remotes/origin/dev/br-2   ce2a804 update file-a
  remotes/origin/master     2f5eaaf add file-a
  remotes/upstream/dev/br-1 7aec18c more updates
  remotes/upstream/master   dc45a94 update file-a from main