You just connect to an IRC network and join any random name you want and the channel is automatically created. However is is recommended that you register the channel afterwards with chanserv to ensure you retain moderation rights to it.

Easy. Join a server like Libera Chat and if you wanted to create a room called randoom you would type in "/join randoom", without quotes of course. And if the channel name isn't taken, then that's it! If not, you need to try again by using a different room name.

Libera.chat IRC network

From Libera.chat's guide:

Creating Channels.
Choosing the correct namespace.
Before creating a new channel on Libera.Chat, please take a moment to read up on our namespaces policy and ensure that your channel is named appropriately from the beginning. This will avoid disruptions in the future.

How to create a new channel.
Be identified.
You must be registered and logged in to services to make channels. You can log in by doing /msg NickServ IDENTIFY YourNickname YourPassword

Joining the channel.
The next step of creating a channel is to /join it.

Pick a channel name without spaces and add the appropriate number of #’s to the beginning, according to our namespaces policy. Eg, /join #awesomeprojectname if it is for an official project channel or a community channel about a project, or /join ##yournickname-chats for an informal channel.

Check that it is available.
You can check that a channel is registered or not with /msg ChanServ info #wanted-channel-name

If it is not registered ChanServ will tell you, otherwise it will show you the public information for the existing channel.

The first person to join an unregistered channel is granted temporary operator status. If the channel is empty and unregistered, you should have operator status now. If not, you will need to talk to the people already in the channel, or if there is nobody there with Libera.Chat staff either in #libera or on /stats p.

If the channel is already registered and you are unable to claim it, you will need to choose a different channel name.

Registering the new channel.
Once you’re sure you can register the channel, you need to /msg ChanServ register #wanted-channel-name

https://libera.chat/guides/creatingchannels