Imparfait vs Passé Composé

Nick, 

it is very common in French to use both tenses in a narrative, often in the same sentence, as you have noted - it is a fundamental component of French grammar. The tense used has significance and conveys meaning/context as the links below explain further.

Takes time, exposure and practice to develop !

There are many sources on the net covering this and a couple of the external links below should help, as well as the lessons from this site :

See lesson below and follow its links for details : 

Using the compound tense (Le Passé Composé) on its own or with the imperfect tense (L'Imparfait) 

A few of the many excellent external links that address this question :

 https://www.lawlessfrench.com/grammar/passe-compose-vs-imparfait/

https://www.frenchtoday.com/blog/french-verb-conjugation/passe-compose-versus-imparfait/ 

https://www.youtube.com/watch?v=3rpQ5xeFneg