Generation I
On the turn that Dig is selected, the user will dig underground and become semi-invulnerable, where the only attacks it cannot avoid are Bide, Swift, and Transform. On the following turn, Dig will do damage, PP will be deducted from it, and it will count as the last move used. Once Dig is selected, the user will not be able to switch out until it is disrupted or fully executed.
If Dig is not fully executed, PP will not be deducted from it, and it will not count as the last move used. If the opponent uses Mirror Move during the turn that the user digs underground, Mirror Move will copy the move that the user executed immediately before using Dig (or fail if it cannot).
Full paralysis and self-inflicted damage due to confusion will disrupt Dig. Additionally, if the user is fully paralyzed during the mostly-invulnerable turn of Dig, the semi-invulnerable part will not be reset until the user switches out or fully executes Dig.
In Pokémon Stadium, Dig will allow the user to avoid Bide. Full paralysis will reset the semi-invulnerable turn of Dig. Mirror Move will copy Dig on either of the turns it takes to execute.
Pokémon can be caught during the semi-invulnerable stage of Dig.
Dig has a power of 100 in Generation I.
Generations II and III
The user can now be hit by Earthquake, Magnitude, and Fissure during the semi-invulnerable turn, and will receive double damage from Earthquake and Magnitude.
The user may also be hit underground if it was previously targeted by Lock-On or Mind Reader. Swift, Bide, and Transform no longer hit on their own during the semi-invulnerable turn, and damage from Sandstorm and Hail will not occur.
Dig's power changed from 100 to 60 in Generation II.
If the user is infatuated during the semi-invulnerable turn, it may fail to execute this move.
In Generation II only, due to a glitch, when Lock-On or Mind Reader are in effect, the moves Attract, Curse, Foresight, Mean Look, Mimic, Nightmare, Spider Web, and Transform cannot hit targets in the semi-invulnerable turn of Dig, and moves cannot lower stats of targets in the semi-invulnerable turn of Dig (status moves such as String Shot will fail, and additional effects of moves such as Bubble will not activate).
Generation IV
Dig's power changed from 60 to 80.
Pokémon can no longer be caught during the semi-invulnerable stage of Dig.
The user may now be hit while underground if it is attacked by a Pokémon with No Guard or has No Guard itself.
If a Power Herb is held by the user, Dig can be executed in one turn.
Source
Pokemon using Dig can be hit by Earthquake, Magnitude, Fissure, a move affected by Lock-On/Mind Reader, or a move used by a Pokemon with No Guard.
Generation I
On the turn that Fly is selected, the user will fly up high and become semi-invulnerable, where the only attacks it cannot avoid are Bide, Swift, and Transform. On the following turn, Fly will inflict damage, PP will be deducted from it, and it will count as the last move used. Once Fly is selected, the user will be unable to switch out unless Fly is disrupted or fully executed.
If Fly is not fully executed, PP will not be deducted from it, and it will not count as the last move used. If the opponent uses Mirror Move during the turn that the user flies up high, Mirror Move will copy the move that the user executed immediately before using Fly (or fail if it cannot).
Full paralysis and self-inflicted damage due to confusion will disrupt Fly. Additionally, if the user is fully paralyzed during the semi-invulnerable turn of Fly, it will not be reset until the user switches out or fully executes Fly.
From Pokémon Stadium onward, Fly will allow the user to avoid Bide. Full paralysis will reset the semi-invulnerable turn of Fly. Mirror Move will copy Fly on either of the turns it takes to execute.
Pokémon can be caught during the semi-invulnerable stage of Fly.
Generation II
The user can now be hit by Gust, Thunder, Twister, and Whirlwind during the semi-invulnerable turn of Fly, and will receive double damage from Gust and Twister.
The user may also be hit in the air if it was previously targeted by Lock-On or Mind Reader, and then struck the following turn, unless said move was Magnitude, Earthquake, or Fissure. Swift, Bide, and Transform no longer hit on their own during the semi-invulnerable turn of Fly.
In this generation, due to a glitch, when Lock-On or Mind Reader are in effect, the moves Attract, Curse, Foresight, Mean Look, Mimic, Nightmare, Spider Web, and Transform cannot hit targets in the semi-invulnerable turn of Fly, and moves cannot lower stats of targets in the semi-invulnerable turn of Fly (status moves such as String Shot will fail, and additional effects of moves such as Bubble will not activate).
Generation III
The user can now also be hit by Sky Uppercut during the semi-invulnerable turn of Fly.
Magnitude, Earthquake and Fissure will now hit a Pokémon using Fly as usual when the target is under the effect of Lock-On or Mind Reader.
Generation IV onwards
Fly's base power was increased from 70 to 90. The user is now unable to avoid moves used by Pokémon with the Ability No Guard during the semi-invulnerable turn of Fly, and unable to avoid moves if it has No Guard. It can also be hit with Hurricane, Smack Down, and Thousand Arrows. If Smack Down or Thousand Arrows hits, the user of Fly will be knocked down, cancelling the move.
If a Power Herb is held by the user, Fly can be executed in one turn.
Fly can hit non-adjacent opponents in Triple Battles.
Source
Pokemon using Fly can be hit by Gust, Thunder, Twister, Whirlwind, Sky Uppercut, Hurricane, Smack Dawn, Thousand Arrows, a move affected by Lock-On/Mind reader, or a move used by a Pokemon with No Guard.
Hope this helps! :)
