How do I show different groups of my products in separate sections?

Make a category for each group in Store Settings, then file each product under its category in Products.

On the page, add a Store Products Grid block for each group, set Source to Your products, and pick the group in Merch Category. For example, one grid for festival merch and another for your everyday line.

Give each grid a Heading and, if you like, an Intro: one line under the heading saying what the group is. The heading, the line and the products then read as one section.

When the grids are groups inside one larger section, such as several partner shops under one Affiliate Store heading, set each grid's Heading Size to Subsection. Its heading then shows smaller than the section titles, so a group reads as part of the section above it. Offer Grid blocks have the same setting.

A grid with nothing to show a visitor leaves the public page, heading and Intro included. That is the case for your own products until Stripe is connected under Payments: they show on your preview address and stay hidden on your real domain. A heading that should show either way belongs in a text block above the grid.