For what it’s worth, here are some remarks about how Wikipedia welcomes newcomers, or rather too often scares them away. This has been problematic for a long time, and afaik (*) it still is.
In early days Wikipedia was very much welcoming towards new contributors. ‘Be bold’ was the mantra to encourage newbies. “Be bold, if you make a mistake, others will correct it. If your contribution is incomplete, others will supplement it.” 
That attitude has long been gone, partly out of necessity. The number of policies and guidelines is astounding.  (often referred to in mailing lists by acronym , which is another way to exclude uninitiated outsiders)
These days the avalanche of incoming edits makes it really hard for recent changes patrollers to keep pace. Some experienced and responsible wikipedians made it their specialty to take care of this. Which is great, and Wikipedia couldn’t do without them.
But the burden to keep pace with the daily onslaught of contributions  brought burnout to some of the most prolific patrollers. They fell victim to short temper. It’s hard to explain a Wikipedia rule in detail if you already did so 10 times today. Perhaps some were also too proud of their own knowledge of those intricate rules. Hence the phrase which is heard too many times: “Don’t bite the newbie”. 
With all incoming edits it is really hard to sift out the troll edits, the silly jokes, the vanity edits, the clueless edits and still remain patient, emphathic, explanatory and supportive. These days a well-meaning newbie too often runs the risk of being shunned away, for instance when his/hers well-meant contribution gets ‘speedy deleted’  (= without due process, or vote).
The more constructive responses which ease the burden are partly by automation, partly social.
Automation: There are tools now to reject many edits automatically. Edits that follow a certain pattern, e.g. edits which replace a full article with a known swear word can be undone without anyone seeing it.
Some articles can only be edited by users a few days after they registered.
Edit patrollers have tools to ease communication and synching up between themselves.
Social: In Dutch Wikimedia workshops for edit patrollers have been held to help them reach out to newbies, while still preserving their good spirits.
In the English Wikipedia community experienced wikipedians can offer their services via a safe place for newbees, the ‘Tea House’.  A similar initiative was discussed in Dutch Wikipedia community but never materialized.
Contests can also bring in new contributors, in a niche where they already feel comfortable. A prime example is the yearly photo contest where Wiki Loves Monuments is the first and most well known version of this yearly contest. Supported by UNESCO, Guinness world record holder for sheer size. Retention rate is a recurring theme here.
These are just a few examples.
All the more special that OSM community still manages to treat newbies in a really supportive and friendly manner. Shout-out to user eggie and others.
 3.8 million in English Wikipedia in March 2021
over 156 thousand in Dutch Wikipedia in March 2021
https://nl.wikipedia.org/wiki/Wikipedia:Richtlijnen_voor_moderatoren section Een_pagina_direct_verwijderen
- I joined as a volunteer in 2002. And have been actively involved till 2019.