If this is your first baby and you don't use daycare/ nursery

She’ll be starting preschool in September but we go to lots of play groups. My cousins are all aged between 3-9 as well and she sees them all the time. We also go to a family pub (social club but not sure if you’ll know what that is since you’re US) and loads of kids go there and she plays with them all the time

Indoor plays, libraries, playgrounds, kid museums, sports and church/community events . Kid friendly things. Also not sure how old your child is but a lot of homeschool groups meet up and have events as well.

Depends on the age. I feel that before 2 (maybe even 3) they don't need to 'be socialised'. All they need is their mum, really.
Exposure to other kids is good of course but just on the playground I've always found enough so far

We go to different playgroups or classs at least 3 times a week x