The Arab slave trade defines a broad swath of history, including slaving in sub-Saharan Africa, but from the early 1500s to 1816 (when the Barbary Treaties were signed) Muslim slavers from North Africa captured European ships and ran a whole system of slaving by piracy. After the Reconquista of Spain and the beginning of the breakup of the Hafsid Empire Spanish Catholics and the Ottoman Empire both saw an opportunity to take parts of the North African coast. The Spanish conquered several Algerian ports, so the Ottomans began sponsoring privateers who mainly operated out of Algiers. You may have heard of Barbarossa, (real name Aruj), who was one of the first of these Muslim pirates. In about 1519, Muslim pirates began capturing ships and raiding European coastlines seriously, as opposed to the mild skirmishes at sea that had taken place for hundreds of years.

Religious terms are more appropriate here than racial ones, primarily because there were “white” Muslims and Arab Christians, who are out of the scope of Muslim North Africa geographically but would have been known about. The people being captured by means of piracy were West European Christians. Before piracy became a big deal, Christian people were enslaved mainly from the Caucuses, by being sold by their relatives to slavers or kidnapped by slavers. Most of these vulnerable people were children. It was extremely costly and hazardous to recover these slaves, so many were lost and grew up serving wealthy Muslims.

Slaves captured by pirates were much more likely to be adults. Think about the people on an average ship back then: some noblemen and clergymen were captured alongside sailors. Many women were captured in coastal raids. These slaves also had a much higher chance of freedom. People in Europe talked extensively about slavery and even started charity organizations through the church tasked with raising money to pay slaves’ “ransoms,” or prices. Muslim pirates encouraged people to pay their family members’ ransoms by playing on the Christian fears of their relatives. It was dreaded that these slaves would end up converting to Islam once in Muslim hands, damning their souls to eternal torment. Muslim slave owners threatened their slaves with selling them deeper into the interior or sending them to work on the galleys, both places where they would be worked very hard and have little contact with relatives. Therefore, this put pressure on families to free their relatives early, before they died or were lost. For Muslim pirates, ransom was profitable. Many times their boats were full on the journey home and they would need to abandon captives anyway, so they used ransom to make money off these spare men and women. Once back in Algiers, sometimes a slave’s ransom was worth more than their labor, especially for noblemen or clergymen, who had extremely high ransoms. This new system and the increased contact between family members contributed to (relatively) many slaves being freed.

If not freed, many slaves died in the galleys, where the work was hard, there was little sleep allowed, and little food. Public works was also a hazardous job to have, and many lower-class Europeans ended up in the bagnos, or slave quarters, slaves of the Sultan and working in quarries.

Some slaves bought their freedom and converted to Islam. Some of these “renegades” even went on to captain gallies of their own. However, many of these men were never truly accepted as a Muslim believer and shunned in the Christian community. Many were poor.

Slaves’ children were not enslaved because either they had none, or they were women married to a Muslim man. Many woman slaves were incorporated into a Muslim man’s harem, and then on the birth of a son married. These children were treated as the man’s legitimate sons, or at least were not slaves. Since the women were put into harems, there were no women working in the bagnos and no chance of children.

As to your last question, I’m not sure about Scandinavia, but definitely the Caucuses. Christian slaves were highly valued, but also were common enough that they were not “curiosities”- Christian travelers are not even phased by their presence, though they may pity them.

That’s an overview of slaving and piracy in North Africa (mainly Tripoli, Tunis, and Algiers) from ~1500 to ~1816. An introductory book on the subject is Christian Slaves, Muslim Masters, though it’s heavy on European perspectives, tries to advance an argument comparing this slave trade to the transatlantic slave trade, and just overall has to be looked at critically.