While browsing assets on the All assets page, there are various ways you can select multiple assets, and this is useful for:
Adding multiple assets to a Collection
Selecting multiple assets
Individual selection
Hover your mouse over an asset
Click the tick symbol that appears at the top-left of the asset
Select the same tick symbol on any other assets you would like to select
Selecting a range
Hover your mouse over the first asset in the range that you want to select
Click the tick symbol that appears at the top-left of the asset
Hold down shift on your keyboard and select the same tick symbol over the last asset in the range
Selecting all visible assets
If you have performed a search and want to select all assets you can see on the page or within a folder, do one of the following:
Either
Press CMD/CTRL + A on your keyboard
Or
Select one asset by clicking the tick box at the top-left of any asset
Click "Select all xxx assets on page" on the top menu bar