Clara's living room and kitchen, Tinkerballa's bedroom, Vork's shed, and Bladezz garage are all sets based in Felicia Day's real house to cut costs down on production. Venom's bedroom in season 3 was actually the flip-side the room that Tinkerballa gamed from.
The frequent references to Codex's anxiety issues stem from Felicia Day's real life struggle with anxiety. As a coping mechanism, she often writes dialogue for Codex which refers to her own anxiety and fears.
The game that The Guild play is literally called 'The Game', and is even shown illustrated in several signs and on video game boxes in the backgrounds of various seasons. This was to avoid any copyright issues and make the game more general.
Codex and Fawkes relationship was initially penned to last slightly longer than it did, and initially, Codex's keeping their dating a secret from the guild had been a massive plot point for season 4. Later, Felicia felt it would be much funnier to change this to Codex's lying about having a relationship with Fawkes to save face.