@Tockity Great questions, and I know there's a lot to learn, but you'll get the hang of it! I'm going to include some links to information that I've found helpful. I'm not fully understanding your question/description about the roofing, but perhaps if you added an illustration, screenshot, or something along that line it would help me understand?
Roofing:
♦ I found this to be a helpful video on various roofing styles/options: https://youtu.be/d3aNEDX5tTs
♦ A nice Carls Guide on building roofs: https://www.carls-sims-4-guide.com/tutorials/building/roofs.php
♦ Another article on roofing options: https://simsvip.com/2017/11/11/tutorial-using-new-roof-styles-sims-4/
Rooms:
Getting the game to see an area as a room can be done usually by re-tracing the area/room, then clicking the side of the area/room to confirm that the game is now seeing it as a room. Please see this video:
https://youtu.be/cfHo3vsOMcM
Inventory: Not all items can be added to/stored in inventory. Some type of items can be added to Personal Inventory. More types of items can be added to Household Inventory. Certain items can be added to either. It all depends on the item:
♦ Personal Inventory: Many items can be stored in the sim's personal inventory by dragging the item to the small picture of the sim at the bottom left of the screen, dragging the item to the sim on the screen, or dragging it to the small 'personal inventory' icon on the bottom right of the screen (or if you click the icon it will open a pop-up and you can drag them into the open area):
https://i.imgur.com/pNZIqX3.png
♦ Household Inventory: Many items can be stored in the household inventory by entering build mode and selecting the item with a the select/hand and hitting the 'delete' key, or by clicking the small box at the bottom left of the screen, then dragging the item to the area in the pop-up:
https://i.imgur.com/fRZFKxo.png
♦ Some items cannot be added to either inventory.
Relationships: As long as the sims are chatting autonomously or by direction their relationship should be increasing. If you want to control it you can choose things like romantic interactions which increase or decrease the romance bar depending on how intense the interaction is and how well it's received. Same thing goes with the regular relationship bar - some interactions will be poorly received and decrease the relationship (you will see a red "-" sign if this is happening, and a green "+" sign if it is positive). If it gets negative enough you can fight them or 'declare enemy'!
Tip: Multitasking for Social: Sims will increase in social and relationship while chatting, and there are some very handy ways to multi-task and cover this need. For example:
♦ TV: You can have a sim sit on a couch/loveseat next to another sim while they are eating. You can then turn on the TV and have them watch the Cooking Channel. Doing this will allow them to be comfortable (if the couch is comfortable) and relieve stress, they will increase their food/hunger bar, they will chat with the other sim and increase their social bar, they will watch the show and increase their fun plus learn cooking so increase their cooking skill.
♦ I do the same trick by putting two treadmills side-by-side, and sometimes putting a wall-mounted TV in front of them on the wall.
On the wall items showing when the wall is hidden/down... yes and no. I find that some items will be invisible when the wall is down and some will not. It depends on the item. It also sometimes depends on if I increased or decreased the item's size &/or placed the item with MOO (Move Objects On) enabled. Some debug items also will not disappear. Since you are new to the game these are probably non-issues, but you can always test by putting the wall down after placing to see if the item remains. And just for fun, I could mention that sometimes an item messes with me - it shows when the wall is down after I place it, but if I save and leave the lot and re-enter, it then disappears with the wall down. Go figure.
I hope this helps some. If you have more questions about the roofing you're welcome to contact me about it and I can address that more specifically: https://sherigr.com/contact/