What can I use instead of SharePoint designer?

yes PowerApps for designing rich interactive forms is the recommended alternative to SharePoint Designer. Also use Flow to automate workflows.

What replaced SharePoint Designer 2013?

“Microsoft InfoPath is being replaced by SharePoint Lists, Flow, and PowerApps – modern solutions for digitizing traditional company forms, automating workflows, and transforming business processes.”

Can you create a workflow without SharePoint designer?

Yes, you can always create a workflow without having SharePoint designer. As per your requirement, you will need SharePoint designer to build your workflow. Or, You can create a custom solution using Visual Studio, answers to this will help you.