Only gains earned on assets inside an ISA wrapper are not taxed. If you made more than 12,300 in realised capital gains this year, they will be taxed.
You can withdraw as much as you hold in an ISA. If you sell assets that have made more than 12,300 in net gains this year outside an ISA, you will have to pay tax.
@Damiansasha if you use the search bar and enter “ISA @Dao” you will find me answering a whole plethora of question relating to ISA’s that should cover all aspects that may cause confusion.
some of my answers are much more comprehensive, but I am currently heading somewhere and didn’t think to save a bookmark to my best answer I can edit it in later once I get back.