Well, a little good news! While I'm no expert, I have at least some of everything you posted and they're all pretty healthy. I might actually be able to help.
I do mean drains at the bottom of the pot, but also that the soil itself lets water through it. Think of sponges that hold different amounts of water when you squeeze them. Your cactus wants sandy soil so it won't hold much water. Your ivy is less picky but probably wants a peatier soil to keep some moisture around the roots.
Let me know species you have and I'll try to keep helping. Bonsai trees are tricky assholes and I have a love-hate relationship with mine.
Oh, and the moldy (Hello from across the pond!) succulents are a common problem. Let them dry out ALL. THE. WAY. between waterings. The soil should be crumbly. If the leaves just barely start to wrinkle, that's alright as long as you get to them soon thereafter. You can seriously let them go weeks between waterings sometimes. Check out some of my other posts for some of my succulents that are doing well.
Did you reuse your Reddit name or create a new one?
I originally went with the same name but have been using this one now since it's new and the idea of starting fresh appeals to me....
Can anyone recommend any good materials for beginners please? Books, videos, wikis... anything. I really want to learn the basics!
I suck at keeping my plants alive. Help!
I definitely think r/gaming has the biggest banger of a privated notice