I've been a member on GitHub for 8 years now, and, as far as I can remember, have always merged branches via the web interface - believing it to be too hard and too confusing to even attempt merging via the command-line.
So I set out to learn it, and boy where my fears overexaggerated, you just need to run
git merge <branch> from the branch you want to merge into. 🤯
And if you don't want to deal with the clean-up of deleting branches on remote and local, use the github-cli and it's one command
gh pr merge - it accepts more information, such as the pr number, URL or branch to merge, you can view the documentation on merge.
Here's the repo I used to learn how to merge using the command-line containing a detailed README of the steps I took, along with my explanation on how I perceive it to work.