If you are working on something and want to use illustrations to improve its appearance, modified or not, without the need for attribution or cost, you are good to go. If you find unDraw or its illustrations to be in the center of what you are doing (e.g. sell or re-distribute one/some of them, add them in an app), then you probably should not proceed.
All images, assets and vectors published on unDraw can be used for free. You can use them for noncommercial and commercial purposes. You do not need to ask permission from or provide credit to the creator or unDraw.
More precisely, unDraw grants you an nonexclusive, worldwide copyright license to download, copy, modify, distribute, perform, and use the assets provided from unDraw for free, including for commercial purposes, without permission from or attributing the creator or unDraw. This license does not include the right to compile assets, vectors or images from unDraw to replicate a similar or competing service, in any form or distribute the assets in packs or otherwise. This extends to automated and non-automated ways to link, embed, scrape, search or download the assets included on the website without our consent.
Are registered trademarks of their respected owners. Are included on a promotional basis and do not represent an association with unDraw or its users. Do not indicate any kind of endorsement of the trademark holder towards unDraw, nor vice versa. Are provided with the sole purpose to represent the actual brand/service/company that has registered the trademark and must not be used otherwise.