For what is the “.well-known”-folder?

That /.well-known/ subdirectory is defined by RFC 5785 RFC 8615

It is increasingly common for Web-based protocols to require the
discovery of policy or other information about a host (“site-wide
metadata”) before making a request. For example, the Robots
Exclusion Protocol http://www.robotstxt.org/ specifies a way for
automated processes to obtain permission to access resources;
likewise, the Platform for Privacy Preferences [W3C.REC-P3P-20020416]
tells user-agents how to discover privacy policy beforehand.

While there are several ways to access per-resource metadata (e.g.,
HTTP headers, WebDAV’s PROPFIND [RFC4918]), the perceived overhead
(either in terms of client-perceived latency and/or deployment
difficulties) associated with them often precludes their use in these
scenarios.

When this happens, it is common to designate a “well-known
location” for such data, so that it can be easily located.
However, this approach has the drawback of risking collisions, both
with other such designated “well-known locations” and with
pre-existing resources.

To address this, this memo defines a path prefix in HTTP(S) URIs
for these “well-known locations”, /.well-known/. Future
specifications that need to define a resource for such site-wide
metadata can register their use to avoid collisions and minimise
impingement upon sites’ URI space.

The reason that you see access forbidden errors might be the result of a blanket block on requests for hidden files/folders (paths starting with a dot .).
In case you do have useful content in /.well-known, this Q&A may be of interest.

Locations within that directory are then used for specific purposes,

Both of which support a similar purpose, they allow the site operator to instruct a visitor to open the site in an associated app, rather than in the (mobile) browser.

IANA maintains a comprehensive list of assigned well-known locations on www.iana.org/assignments/well-known-uris/well-known-uris.xhtml and a similar list on Wikipedia also includes a few different URI’s that are not officially assigned and registered by IANA.

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