There are three ways of joining a public channel; through
To search for channels and users, use the search bar under your account box. Alternatively you can press Ctrl/Cmd + k to access the search bar quickly.
A search will find channels (including the ones you are part of) and users.
You can not search inside private channels.
When you click on a search result, the following can happen:
If the result is a channel, and you already aren’t part of it, you will join that channel;
If the result is a user, you will start a direct message with that user;