I've been doing HTTPS on hosted domains for years, and I don't really get how it works in this case. Is it that you don't use your own certificate, but GitHub a

I've been doing HTTPS on hosted domains for years, and I don't really get how it works in this case. Is it that you don't use your own certificate, but GitHub automatically generates one with Let's Encrypt for you? If not, then how exactly do you give GitHub your cert? The instructions seem vague on this.