When seen by detect magic, a polymorphed beast will glow magical (Transmutation). Since the creature is magical, do their attacks count as magical for overcoming resistances?
A polymorphed beast has the stats of whatever it has been changed into. Unless the creature it is transformed into has some ability that makes their attacks magical, the creature's attacks are not inherently magical.
For example, a Barbed Devil, does not have any features that make its attacks magical. Nor does the more powerful Bone Devil. However, when we get to the extremely powerful Pit Fiend, we see the feature 'Magic Weapons', which states:
The pit fiend's weapon attacks are magical.
A general rule is that more powerful creatures, or creatures with a lot of magical lore, will exhibit a feature like this. Sometimes it will appear described as something else, like with the Deva's Angelic Weapons:
The deva's weapon attacks are magical. When the deva hits with any weapon, the weapon deals an extra 4d8 radiant...
The key to knowing if a creature's attacks (physical and ranged) are considered magical is some feature in the stat block says they are.
As an aside, I did a quick review of all the beasts available on DNDBeyond and could not find any beasts that had an ability similar to what I've described above. This does not mean that won't change in the future with the release of additional content, but for the purposes of this question, it seems that across the board all beasts' attacks are non-magical.
No: polymorphing into a beast does not make the attacks magical
Here's the reasoning behind that answer.
The polymorph spell turns a creature into a beast:
The new form can be any beast whose challenge rating is equal to or less than the target’s (or the target’s level, if it doesn’t have a challenge rating). The target’s game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
If you can find a beast where the stat block says that it has attacks which count as magical, then the answer will be yes for that beast form. (I have not found one yet, but there are some unusual beasts in published adventures and in Mordenkainen's Tome of Foes that I can't check on just now.)
If the spell did that - make the beast's attacks magical - it would say so in the spell's text.
I point that out because for the druid's Wild Shape feature, which also turns a creature (the druid) into a beast, the 6th-level Circle of the Moon subclass feature Primal Strike does specify that attacks in beast form are magical for the purpose of overcoming resistance/immunity. Because they found the need to specify that, then it is rational to conclude that they would have specified that for the polymorphed creature-as-beast if it were also true in the case of the spell.
But they didn't.