The use of ollas in the garden intrigued me for a long time, but purchasing "real" ollas was cost prohibitive. Four years ago, I found some 8 inch unglazed terracotta pots at the dollar store and decided to try making ollas. To make one olla, I used two pots. The bottom drainage hole in one pot was sealed by gluing a piece of scrap tile over it. Then I inverted a second pot over the one with the sealed drainage hole, and glued the rims together. After testing for water tightness, I buried them in the garden, with 3-4 inches above the soil line. The ollas get filled through the drainage hole in the inverted pot on top and hold about a gallon and a half. I made 6 of them using gorilla glue (per an instructional video found online) but will probably use silicone in the future. Thus far, they have worked very well and the plants growing closest to them definitely look happier than those further away. The plants stay hydrated and happy for several days even during the summer when temperatures are over 100F.
Farmer's almanac says you can put liquid fertilizer in an olla at a reduced concentration, but I only use water in the ollas, and use compost tea or liquid nutrients on occasion to water the soil at the base of the plants. It does not seem like the ollas prevent the plants around them from growing deep roots, but in this area, pretty much the only water the plants get is whatever water they are irrigated with. For what it's worth, transplants are easier/do better around the ollas than seed because you have to water the soil around the ollas where the seeds are until they germinate and develop a reasonable root system, and it's hard to tell when they are ready to draw water from the olla on their own.
Location: southern California zone 9b, hot, dry, high desert