STOCK-BASED AWARDS - Schedule of Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Number of Options
Balance - beginning of period (in shares) | shares	70,675,318
Options granted (in shares) | shares	8,402,925
Options exercised (in shares) | shares	(8,966,272)
Options canceled (in shares) | shares	(3,098,349)
Balance - end of period (in shares) | shares	67,013,622	70,675,318
Weighted Average Exercise Price
Balance - beginning of period (in dollars per share) | $ / shares	$ 1.07	$ 0.84
Options granted (in dollars per share) | $ / shares	2.85
Options exercised (in dollars per share) | $ / shares	0.69
Options canceled (in dollars per share) | $ / shares	1.68
Balance - end of period (in dollars per share) | $ / shares	$ 1.07	$ 0.84
Additional Disclosures
Options outstanding, weighted average remaining contractual term	6 years 9 months 18 days	7 years 9 months 18 days
Options outstanding, intrinsic value | $	$ 1,628,923	$ 647,218
Options vested and exercisable, number of options (in shares) | shares	41,932,824
Options vested and exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0.75
Options vested and exercisable, weighted average remaining contractual term	5 years 8 months 12 days
Options vested and exercisable, intrinsic value | $	$ 1,032,660