While browsing assets on the Browse page, There are three different ways you can select multiple assets at the same time (useful for sharing, bulk editing and bulk downloading):
Selecting a single group of assets
Select the first asset of the group you want to select
Hold down shift
Select the last asset in the group you want to select

Selecting specific assets
Select the first asset
Hold down CMD (on a Mac) or CTRL (on Windows) and select the next asset you would like
Repeat the above until you've selected all the assets you would like

Selecting all assets
Use the search and filters to show the assets you want to select
Select the first asset
Click "Select all x on page" in the menu bar

Alternatively you can use the keyboard shortcut CMD+A (Mac) or CTRL+A (Windows) to instantly select all assets in your current view.