If a user changes stock levels on the channel will Linnworks update these in Linnworks.

No Linnworks will not, Linnworks is the source of truth and therefore all inventory updates should happen in Linnworks, users book their stock in, manage level availability and manage orders from other marketplaces, all these need to be taken into account when updating stock levels.

There are also user mechanisms to control stock such as max listed, end when and stock percentage as well as many locations so it wouldn't be as simple as overwriting the stock level if it did exist.

Will Linnworks send us all stock levels all the time?

No Linnworks will only send relevant change to the channel. For example when an order has downloaded from a different channel and the available level has reduced.

Linnworks assumes that the channel decrements levels for it's own orders and therefore we will not push stock levels up for these orders.

Is there a way to resync all stock levels daily?

Currently this is not possible as it could cause a massive overhead especially where the customer has a large inventory. If this case needs to managed it's suggested a log of all deltas is stored on the application side and handled on an outside automation.