Almost nothing supports ESNI yet. Chrome does not have it yet. Firefox does but it very difficult to enable, there is a config flag but it does nothing on its own unless you also enable DNS over HTTP in Firefox.
OpenSSL has no support for ESNI yet either.
ESNI also never tells the user if it is working or not yet, making downgrades fairly easy.
ESNI is a long way from being deployed, let alone useful.
Users can test their ESNI support online here: https://www.cloudflare.com/ssl/encrypted-sni/
But, I encourage you to try to get it working in _your_ firefox, where that site says you are in fact using ESNI. It is trickier than just enabling a flag.
If you think this means people are using it, sure. Firefox has ~9% of market share, and a very small percent (maybe 0.1%) of those users have correctly configured this, and when those users visit a page hosted by cloudflare they will may be use ESNI. That is in fact more than zero users. It is a very very small number today, but hopefully it will grow in the future.
And while that page you link tests ESNI, I cannot verify that any other domain supports ESNI. I am unaware of any tooling that lets me test that today.
Only if you use their built-in DoH resolver. The public keys for ESNI are distributed by DNS records and as I understand it there's a bunch of work to be done for Firefox to retrieve these records from classic DNS servers. That work has been classified P5 (we won't do it, but might accept patches)[0].