Applying silver does not make a weapon magical.
The rules for silvering weapons read:
Some monsters that have immunity or resistance to nonmagical weapons are susceptible to silver weapons, so cautious adventurers invest extra coin to plate their weapons with silver. You can silver a single weapon or ten pieces of ammunition for 100 gp. This cost represents not only the price of the silver, but the time and expertise needed to add silver to the weapon without making it less effective.
Nothing here says the weapon becomes magical, so a nonmagical weapon does not become magical. Monsters that are susceptible to silvered weapons will have that annotated in their stat blocks. For example, the Bone Devil:
Damage Resistances Cold; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Silvered
A silvered weapon overcomes the Bone Devil's resistance to nonmagical weapons because the resistance text explicitly states that it is only resistant to attacks from weapons that are not silvered.
In contrast, the Rakshasa's immunity reads:
Damage Immunities Bludgeoning, Piercing, and Slashing from Nonmagical Attacks
Since a silvered weapon delivers a nonmagical attack, and no exception is made for silvered weapons in the immunity text, the Rakshasa is totally immune to bludgeoning, piercing, and slashing damage from silvered weapons, unless the weapon just happens to also be a magical weapon. Applying silver to a weapon does not make it magical, but silver can be applied to a magical weapon, yielding a magical silver weapon.