On web, scroll down on tea spot, till you find Channels, click join channels, you would see all the public channels on the platform (note, you do not need any approval to join a public channel),
Simply click join channel for any channel of your choice.
On mobile, access the side menu to view channel, the above step applies
You can also view and join channel from a company’s public page, and search results page (the channel has to be a public channel)
Note: you can only be invited to a private channel by the channel Admin