The nodraw texture is used on any brush or brush face that you don't want rendered. Any brush face that the player won't ever see should be textured with this to reduce the number of brush faces rendered. Nodraw shouldn't be used where visible to the player, with some exceptions, as nodraw does block
visibility. If visible to the player, areas behind it may be glitchy or not visible at all; if nothing is seen behind the nodraw, players will either see the skybox or a
hall of mirrors effect.
To make an invisible wall, you have various textures at your disposal. The clip or playerclip textures are used to block players from accessing parts of the map they shouldn't be getting to, such as rooftops or blocked-off hallways. These textures are solid to players, but not bullets, projectiles, or other stuff. Both clip and playerclip function identically in TF2. (In other games, playerclip is not solid to NPCs. In TF2, the only NPCs are TFBots and halloween bosses, but those are treated as clients rather than as NPCs, so playerclip is still solid to them.) The clip or playerclip textures are also commonly used with railings to allow players to easily shoot through the rails.
If you need something that is solid to everything, blockbullets or blockbullets2 are both solid to players, bullets, projectiles, and everything else AFAIK. The only difference is that blockbullets cuts
visleafs, so blockbullets2 is better. One popular use for blockbullets is creating collisions for props that lack collision models. There are also tons of other tool textures that you can read about
here.