They appear as beige credits with a brown line in the middle. There is the Pixel Gun.
They are only used to upgrade the Battle Pass.
They can be obtained primarily by clicking on the challenges and tasks of the days that were missed out. Around 10 battle credits will be earned. They can also be obtained by playing matches and best in brawls, since the credit rewards are usually multiplied.
These credits can only be spent on upgrading the Battle Pass, priced at 2400 battle credits. Another option is also upgrading the Battle Pass, but skipping 15 levels, priced at 6000 battle credits, but there is always a sale that reduces the price to 4800 battle credits.