Hi and welcome!
Prefabs: You can save anything you build by selecting it and clicking "create prefab" on the right side. It will save it then in the prefabs folder. Put any prefab you download into the same folder and you can use them from there.
CS Startpositions: info_player_terrorists and info_player_counterterrorists
You would also need a buyzone for both. Make a brush where the buyzone should be, put the "trigger" texture on all sides. Tie it to entity "func_buyzone" select the team which can buy there in the properties.
Tutorial on custom textures (didn't make my own textures yet so I don't know much about it):
http://www.snarkpit.net/editing.php?page=tutorials&game=HL2&id=226Those portals with triggers: Make the bruh and tie it to entity "trigger_multiple". Apply changes. Add outputs (in the properties) that do what you want. Don't know in detail how a portal would be done with a trigger but I imagine it's possible with them.