In general fluid. In the last major Europe wide war of the seven years war France had made an alliance with Austria against the Prussians ant British but no particular set of alliances were at all stable in the 18th century.

The alliances of the period prior to the revolutionary wars were marked by what was referred to as the stately quadrille because the four western major powers of the day, Britain, France, Austria and Prussia would often swap alliances.

One of the very few consistencies to my mind is that Britain and France would almost always end up on opposing sides of major conflicts.

As a very general rule Prussia and Austria usually ended up opposing each other although that was less true earlier in the 18th century when they fought alongside each and Britain against the French with assorted smaller nations on either side during the Spanish War of succession.

But as after that war and the earlier 30 years war the Holy Roman Empire's power had been in decline and Prussia as a rising power began to contest the Austrian based Habsburgs for control over the Holy Roman Empire and in particular Silesia. However exceptions to this rivalry would be made on occasions Prussia, Austria and Russia decided to join forces to bully Poland because they had nothing better to do i guess.

Likewise France was also usually a rival of the Austrian Habsburgs contesting over things like the throne of Spain, the Spanish/Austrian Netherlands/modern Belgium and in the Italian States. Notably in addition to the already mentioned Spanish War of succession they would face off against each other in the Austrian War of succession when France allied with Prussia and Britain allied with Austria. But as i mentioned earlier France allied with their historic rivals the Austrians while Britain who had been unhappy with Austria's performance in the war of Austrian succession allied with Prussia.

Aside from this the fifth and sixth wheels of Spain and Russia. Spain was both no long habsburg and not the great power it had once been. In the last two wars of note in the peroid, the 7 years war and the American war of Independence Spain and France had allied together to fight the British and had a decent relationship still. Russia meanwhile often allied with Austria, partly due to their mutual rival in the Ottomans, as it did for parts of the Austrian war of succession and 7 years war. But while it could raise powerful armies had a tendency to withdraw from conflicts following a major set back meaning that in wars like the war of Austrian succession and 7 years war they sometimes dipped in and out of wars.