How do they work and what are PTR records for?

Question

Mail server

I'm trying to figure out how the Internet works, and in particular E-Mail
. I came across an article on PTR records, and after reading it, only more questions appeared.
1. The IN-ADDR.ARPA domain is a real physical domain that is located somewhere and maintained by real people, or is it an abstract name that is just reserved for use for E-Mail, along with 192.168..., 172.27... and 127.0.0.1?
2. Why are IP addresses written in reverse order in PTR records? What's the point of this?
3. Is there any software that allows you to simulate networks, say, raise several virtual machines, and set up different roles for them. For example, configure one as a provider, the other as just a computer, etc., in order to visually simulate interaction on the Internet, and no longer ask similar questions here

Answer 1

1. The IN-ADDR.ARPA domain is a real physical domain that is located somewhere and maintained by real people, or is it an abstract name that is just reserved for use for E-Mail, along with 192.168..., 172.27... and 127.0.0.1?

in-addr.arpais a real existing domain. PTR is a DNS concept, it has only an indirect relation to email (email uses ptr records for ip addresses).
When you traceroute to an IP address, the domain names you see in the output are obtained by querying the PTR record of the IP address:

traceroute to ya.ru (87.250.250.242), 30 hops max, 60 byte packets
 1  _gateway (192.168.101.1)  49.797 ms  119.433 ms  119.416 ms
 2  hosted-by.yourserver.se (95.215.45.5)  119.308 ms  119.242 ms  119.179 ms
 3  stk-tct-cr3.be22-1841.rascom.as20764.net (81.27.252.200)  166.091 ms  165.839 ms  165.776 ms
 4  * * *
 5  178.18.226.119.ix.dataix.ru (178.18.226.119)  165.459 ms  165.345 ms  165.312 ms
 6  * ya.ru (87.250.250.242)  79.669 ms *

PTR was perhaps the first technology to protect against spam from hacked servers or untrusted networks, but only because it is "non-trivial" to change it in the general case when the sender does not have the authority to do so.

2. Why are IP addresses written in reverse order in PTR records? What's the point of this?

Ease of storing the "reverse" tree in the memory of the DNS server, fast search for records. Rather, a technological feature of the 80s, and not a necessity.

3. Is there any software that allows you to simulate networks, say, raise several virtual machines, and set up different roles for them. For example, configure one as a provider, the other as just a computer, etc., in order to visually simulate interaction on the Internet, and no longer ask similar questions here

Depends on what exactly you need to model. See Cisco Packet Tracer, gns3.

Answer 2

The PTR record is made on the side of the IP owner and must point to the hostname of the server.
On the DNS side of the domain, there must be an A-record (hostname) for this IP.
Logic from the name Reverse DNS or reverse DNS record.

Answer 3

1. https://ru.wikipedia.org/wiki/%D0%9E%D0%B1%D1%80%D...
is used to confirm that the IP address really belongs to the domain, for example hosting or home provider will not give you register a PTR record for the IP address.
2. see item 1
3. Cisco Packet Tracer, GNS3