Hi @BlueWhaleYT, welcome to our community!
@Dravins, @Suzzy-Grey and @kenobi are all experts here. The conflicting advice from them stems from the different behaviour of objects depending on the mechanism used to animate or control them.
For the use case of an object controlled by the control pad colliding with another object, set them both to solid (the default). Here’s an example that does what I understand you to be after. (Here, the white ball is the player, the red ball is the enemy. The player is controlled by the control pad and the enemy has reciprocating bullet behaviour that causes it to travel back and forth.)
note: if you open the link on a mobile device, it will open the project in Ready Maker where you can edit it.