How do they block private servers?

Different Types of private server Blocks
IP Blocking
Your IP address shares a lot of information about your device and location with the websites you visit. If you‘re using a private server
, websites will see the IP address and location of the server you‘re connected to instead.
Companies like Netflix and Hulu compare their visitors’ IP addresses against a database of a known private server and will block your access to their website if there’s a match. Many private server providers regularly change their servers’ IP addresses to evade this discovery as newer IPs won’t yet be on the blacklist.
Port Blocking
When you use a private server, your data passes through a number of ports. Network managers commonly close the ports that are most often used by private servers so that they cannot access the network.
Deep Packet Inspection (DPI)
Governments and companies can use DPI techniques to examine the data that travels through their network and act depending on what they find. Using DPI, they can to decide if a private server is being used and then either block, re-route or log the data.

How to Bypass Private server Blocks at Work
Change your private server port: If your network‘s manager has identified and closed the ports that private servers most often use on your network, you may be able to configure your private server to use a lesser-known port. Some good ones to try are 2018, 41185, 443, and 80.
Use a private browser: private browsers are the secure browsers that mask your IP address. they allow users to access and share online content anonymously and can be a great tool if you‘re looking to bypass a private server block. You can download many of them for free, but if you’re at work, maybe you will need admin rights on your device to install them.
Private browsers pass your data through their network of servers, which prevents network administrators from tracking your activity. Also with some of them, you’ll be able to prevent DPI detection, too.

reanson:
Different Types of private server Blocks
IP Blocking

Use a private browser: private browsers are the secure browsers that mask your IP address. they allow users to access and share online content anonymously and can be a great tool if you‘re looking to bypass a private server block. You can download many of them for free, but if you’re at work, maybe you will need admin rights on your device to install them.
Private browsers pass your data through their network of servers, which prevents network administrators from tracking your activity. Also with some of them, you’ll be able to prevent DPI detection, too.

So you mean if I am accessing a site with a private browser on chrome or Firefox, the IP address would be undetected? I don't think so.