In GTA San Andreas on Android, you can legally enter the other two cities, San Fierro and Las Venturas, by progressing through the game's storyline. Here's when you can access them:
: Becomes accessible after completing The Green Sabre mission. This mission typically occurs about 20% into the game56.
: Becomes accessible after completing Yay Ka-Boom-Boom mission. This mission usually occurs about 50% into the game56.
If you want to access these cities earlier, you can use mods like the "Unlock City" mod, which removes roadblocks but will give you a four-star wanted level37. Alternatively, you can use cheats to bypass these restrictions, but this is not a legal in-game progression method4.