According to DNSCrypt Wiki:

These are normal, non-encrypted DNS resolvers, that will be only used for one-shot queries when retrieving the initial resolvers list and if the system DNS configuration doesn't work.

No user queries will ever be leaked through these resolvers, and they will not be used after IP addresses of DoH resolvers have been found (if you are using DoH).

They will never be used if lists have already been cached, and if the stamps of the configured servers already include IP addresses (which is the case for most of DoH servers, and for all DNSCrypt servers and relays)

They will not be used if the configured system DNS works, or after the proxy already has at least one usable secure resolver.

Resolvers supporting DNSSEC are recommended, and, if you are using DoH, bootstrap resolvers should ideally be operated by a different entity than the DoH servers you will be using, especially if you have IPv6 enabled.

Put valid standard resolver addresses here. Your actual queries will not be sent there. If you're using DNSCrypt or Anonymized DNS and your lists are up to date, these resolvers will not even be used

InviZible uses bootstrap resolver as parameter for DNSCrypt netprobe_address:

Address and port to try initializing a connection to, just to check if the network is up. It can be any address and any port, even if there is nothing answering these on the other side. Just don't use a local address, as the goal is to check for Internet connectivity. On Windows, a datagram with a single, nul byte will be sent, only when the system starts. On other operating systems, the connection will be initialized but nothing will be sent at all.

In addition, InviZible uses a bootstrap resolver to detect Internet connectivity to display the connection status in a notification when Tor is not running. So, you should use any real DNS resolver anyway. Otherwise DNSCrypt and InviZible may not work correctly.