Whenever Spencer needs to go shopping he invites Penelope to keep him company. Realistically he can get shopping done pretty quickly but with Garcia she'll stop at literally the smallest thing and then she'll say they should go get lunch and then what do you know, they go back home with a shit ton of stuff and it's 11 pm
Since they're both neurodivergent they play off each other so much. Like they go back and forth at hanging out at each other's apartments and anyone else would talk about how cluttered their homes are. But Spencer is like "wow this pillow has a great texture!" And Garcia is like "!!! So many books!!! Tell me about them all!!"
Personally i hc that her parents knew she was nuerodivergent and they were so proud of her they made sure she knew she was loved and that she should always be herself. They always let her hyperfixate and they gave her a shit ton of fidget toys so she would be stimulated. Spencer was the opposite. His parents didn't know and looking back he's glad they didn't he was already treated so poorly in childhood.
Even tho when Spencer joined the bau and he didn't tell anyone he had autism Garcia could just tell. She was constantly giving him gifts. They trade fidget toys and advice and hyperfixations.
Garcia also could tell that Spencer's childhood was shitty so she makes sure that her apartment was always a safe place. Always yummy food and things to stim over. Even the smallest of things like blankets and pillows and other stuff.
Spencer has been masking for all of his life so whenever Spencer talked about "oh i really like the way this type of fabric feels-" she is ON IT! She makes him as much clothes as he'll allow and she knit him a pride weighted blanket.
Garcia is constantly sharing her special interests and hobbies and most of them Spencer adapts. Like Garcia is talking about all her plants and then the next time Morgan goes over to Spencer's the place is COVERED with plants and Morgan is like "wtf" and Spencer is like "IT WAS GARCIA!"
Garcia notices that Spencer doesn't talk about his interests so she always asking him about it and his face lights up everytime. She lets him talk ALL DAY. She pulls out the tea and sits there and looks at him. Turns out, he has a collection of Disney movies on VHS
On their day off they sit on his couch and cuddle while they watch Disney movies. Penelope says she's cold and he IMMEDIATELY untangles his limbs and her a blanket. She jokes "such a gentleman" he smiles. When he tries to get more comfortable and she's like "aw poor genius with his long legs"
Since Garcia makes him the most comfortable than anyone ever he's ok with her touching him. He's ok with the cuddles and the hugs and it gives him comfort. So whenever they're in public and his surroundings are too much he'll just grab her hand and book it or if he's feeling bad he'll kiss her cheek or forehead.