Multiple Side Gigs

In general, you need to separate your businesses and file a separate schedule C for each.  This depends mostly on the type of work and the expenses involved.  If you did rideshare, meal delivery and grocery delivery for several companies, you could combine them because the work (driving) and the expenses (mostly mileage) are basically the same.  If you did rideshare and landscaping, you would keep those separate since they have different type of work and different kinds of expenses.

Itemizing your personal deductions has nothing to do with business expenses.  You report business income and expenses on schedule C (more than one if you have different jobs) and all the net income flows to the form 1040, where it is combined with any other income like wages or investments.  Then your personal deductions and credits are taken into account to determine your final income tax.