Mastodon has two different official web interfaces: single-column (default) and multi-column (advanced).
By default you will see the single column interface, but you can optionally switch over to the Tweetdeck-style multicolumn one if you prefer. This lets you view lots of timelines at once.
There are complete instructions on how to activate it at:
➡️ https://fedi.tips/how-to-activate-mastodons-tweetdeck-style-advanced-web-interface
The guide also answers lots of questions about how to use it and customise it.
