It’s not really that consistent. Create bonfire, for instance, specifies that the bonfire is magical. Sage Advice has this to say on what does and doesn’t count as magical:
Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:
• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell
that’s mentioned in its description?
• Is it a spell attack?
• Does its description say it’s magical?
If your answer to any of those questions is yes, the feature is magical.
This doesn’t really tell us how to determine whether something a spell
produces is magical (unless the description specifies that it is, like in the case of create bonfire), but Sage Advice also tells us:
Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend. Here’s why: the effects of an instantaneous spell are brought into being by magic, but the effects aren’t sustained by magic (see PH, 203). The magic flares for a split second and then vanishes. For example, the instantaneous spell animate dead harnesses magical energy to turn a corpse or a pile of bones into an undead creature. That necromantic magic is present for an instant and is then gone. The resulting undead now exists without the magic’s help. Casting dispel magic on the creature can’t end its mockery of life, and
the undead can wander into an antimagic field with no ad- verse effect.
Ice storm’s duration is instantaneous, which means the ice it creates can’t be removed by dispel magic, and persists in an antimagic field. So I’m inclined to say it is not magical, and land’s stride can ignore it. On the other hand, spike growth’s duration is concentration, up to 10 minutes. The difficult terrain it creates can be dispelled by dispel magic and antimagic field. Therefore, I’m inclined to say it is magical, and land’s stride can’t ignore it.