I am looking for a simple app for filling forms

The free Adobe Acrobat Reader DC can fill forms, as can to a limited extent, Apple's Preview application. You may need to save as since neither are PDF Editors and cannot save the form-filled PDF over the original.

If the PDF form also has Adobe Acrobat JavaScript (AcroJS) associated with a form field to control or validate its entry, then Apple's Preview will not recognize that AcroJS functionality. You may need to use Apple's character panel (control+command+spacebar) to access some of the checkmark or circle characters.

I want an app that resides on the Mac computer and not online.

I understand. The intended recipient of the forms you propose to create is also an important question that needs to be addressed.

Whereas there are many options for form creation, if your target audience is reluctant or unwilling to download, install or use some third-party product on their own personal Macs you are likely to arrive at the conclusion Adobe Acrobat or whatever similar bloatware they have to offer will be required to create those forms. In that case Preview is perfectly suitable for completing them, to subsequently print, email them, and so forth.

On the other hand if your forms are to be created and deployed among Macs under your personal control in which you can install whatever you want, less expensive options are almost certain to exist.

Who better to provide PDF compatibility in their PDF reader than Adobe. Apple probably won't improve Preview due to the Adobe licensing costs and the traumatic upheaval to all Apple and third-party applications that use Apple's PDFKit framework.